X11: Fixes for OpenGL 3.0 and later context creation.

- Don't create a temporary context first; this was probably due to Windows
needing one to get the address of wglCreateContextAttribsARB(), but that's
a unique quirk of WGL, and doesn't apply to glX. The glX spec explicitly says
you have to get a function pointer that works with any context from
glXGetProcAddress(), including when no context exists.

- Properly check for the GLX_ARB_create_context instead of just looking for a
non-NULL glXCreateContextAttribsARB()...some implementations, like Mesa,
never return NULL for function lookups (Mesa returns pointers into a jump
table that is filled out when the GL is initialized; since you can look up
functions before you have a valid context, it can't definitely say a function
isn't valid at that point).

src/video/x11/SDL_x11opengl.c

@@ -373,6 +373,16 @@ X11_GL_InitExtensions(_THIS)
             (int (*)(int)) X11_GL_GetProcAddress(_this, "glXSwapIntervalSGI");
     }
 
+    /* Check for GLX_ARB_create_context */
+    if (HasExtension("GLX_ARB_create_context", extensions)) {
+        _this->gl_data->glXCreateContextAttribsARB =
+            (GLXContext (*)(Display*,GLXFBConfig,GLXContext,Bool,const int *))
+                X11_GL_GetProcAddress(_this, "glXCreateContextAttribsARB");
+        _this->gl_data->glXChooseFBConfig =
+            (GLXFBConfig *(*)(Display *, int, const int *, int *))
+                X11_GL_GetProcAddress(_this, "glXChooseFBConfig");
+    }
+
     /* Check for GLX_EXT_visual_rating */
     if (HasExtension("GLX_EXT_visual_rating", extensions)) {
         _this->gl_data->HAS_GLX_EXT_visual_rating = SDL_TRUE;
@@ -576,7 +586,6 @@ X11_GL_CreateContext(_THIS, SDL_Window * window)
     XVisualInfo v, *vinfo;
     int n;
     GLXContext context = NULL, share_context;
-    PFNGLXCREATECONTEXTATTRIBSARBPROC glXCreateContextAttribs = NULL;
 
     if (_this->gl_config.share_with_current_context) {
         share_context = (GLXContext)SDL_GL_GetCurrentContext();
@@ -601,11 +610,6 @@ X11_GL_CreateContext(_THIS, SDL_Window * window)
             context =
                 _this->gl_data->glXCreateContext(display, vinfo, share_context, True);
         } else {
-            /* If we want a GL 3.0 context or later we need to get a temporary
-               context to grab the new context creation function */
-            GLXContext temp_context =
-                _this->gl_data->glXCreateContext(display, vinfo, NULL, True);
-            if (temp_context) {
             /* max 8 attributes plus terminator */
             int attribs[9] = {
                 GLX_CONTEXT_MAJOR_VERSION_ARB,
@@ -631,48 +635,27 @@ X11_GL_CreateContext(_THIS, SDL_Window * window)
             attribs[iattr++] = 0;
 
             /* Get a pointer to the context creation function for GL 3.0 */
-                glXCreateContextAttribs =
-                    (PFNGLXCREATECONTEXTATTRIBSARBPROC) _this->gl_data->
-                    glXGetProcAddress((GLubyte *)
-                                      "glXCreateContextAttribsARB");
-                if (!glXCreateContextAttribs) {
-                    SDL_SetError("GL 3.x is not supported");
-                    context = temp_context;
+            if (!_this->gl_data->glXCreateContextAttribsARB) {
+                SDL_SetError("OpenGL 3.0 and later are not supported by this system");
             } else {
                 int glxAttribs[64];
 
                 /* Create a GL 3.x context */
                 GLXFBConfig *framebuffer_config = NULL;
                 int fbcount = 0;
-                    GLXFBConfig *(*glXChooseFBConfig) (Display * disp,
-                                                       int screen,
-                                                       const int *attrib_list,
-                                                       int *nelements);
-
-                    glXChooseFBConfig =
-                        (GLXFBConfig *
-                         (*)(Display *, int, const int *,
-                             int *)) _this->gl_data->
-                        glXGetProcAddress((GLubyte *) "glXChooseFBConfig");
 
                 X11_GL_GetAttributes(_this,display,screen,glxAttribs,64,SDL_TRUE);
 
-                    if (!glXChooseFBConfig
+                if (!_this->gl_data->glXChooseFBConfig
                     || !(framebuffer_config =
-                             glXChooseFBConfig(display,
+                        _this->gl_data->glXChooseFBConfig(display,
                                           DefaultScreen(display), glxAttribs,
                                           &fbcount))) {
-                        SDL_SetError
-                            ("No good framebuffers found. GL 3.x disabled");
-                        context = temp_context;
+                    SDL_SetError("No good framebuffers found. OpenGL 3.0 and later unavailable");
                 } else {
-                        context =
-                            glXCreateContextAttribs(display,
+                    context = _this->gl_data->glXCreateContextAttribsARB(display,
                                                     framebuffer_config[0],
                                                     share_context, True, attribs);
-                        _this->gl_data->glXDestroyContext(display,
-                                                          temp_context);
-                    }
                 }
             }
         }

src/video/x11/SDL_x11opengl.h

@@ -40,6 +40,8 @@ struct SDL_GLDriverData
     void *(*glXGetProcAddress) (const GLubyte*);
     XVisualInfo *(*glXChooseVisual) (Display*,int,int*);
     GLXContext (*glXCreateContext) (Display*,XVisualInfo*,GLXContext,Bool);
+    GLXContext (*glXCreateContextAttribsARB) (Display*,GLXFBConfig,GLXContext,Bool,const int *);
+    GLXFBConfig *(*glXChooseFBConfig) (Display*,int,const int *,int *);
     void (*glXDestroyContext) (Display*, GLXContext);
     Bool(*glXMakeCurrent) (Display*,GLXDrawable,GLXContext);
     void (*glXSwapBuffers) (Display*, GLXDrawable);