Fixed crash if initialization of EGL failed but was tried again later.

The internal function SDL_EGL_LoadLibrary() did not delete and remove a mostly
uninitialized data structure if loading the library first failed. A later try to
use EGL then skipped initialization and assumed it was previously successful
because the data structure now already existed. This led to at least one crash
in the internal function SDL_EGL_ChooseConfig() because a NULL pointer was
dereferenced to make a call to eglBindAPI().

test/relative_mode.markdown

@@ -0,0 +1,58 @@
+Relative mode testing
+=====================
+
+See test program at the bottom of this file.
+
+Initial tests:
+
+ - When in relative mode, the mouse shouldn't be moveable outside of the window.
+ - When the cursor is outside the window when relative mode is enabled, mouse
+   clicks should not go to whatever app was under the cursor previously.
+ - When alt/cmd-tabbing between a relative mode app and another app, clicks when
+   in the relative mode app should also not go to whatever app was under the
+   cursor previously.
+
+
+Code
+====
+
+    #include <SDL.h>
+
+    int PollEvents()
+    {
+        SDL_Event event;
+        while (SDL_PollEvent(&event))
+        {
+            switch (event.type)
+            {
+                case SDL_QUIT:
+                    return 1;
+                default:
+                    break;
+            }
+        }
+
+        return 0;
+    }
+
+    int main(int argc, char *argv[])
+    {
+        SDL_Window *win;
+
+        SDL_Init(SDL_INIT_VIDEO);
+
+        win = SDL_CreateWindow("Test", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 800, 600, 0);
+        SDL_SetRelativeMouseMode(SDL_TRUE);
+
+        while (1)
+        {
+            if (PollEvents())
+                break;
+        }
+
+        SDL_DestroyWindow(win);
+
+        SDL_Quit();
+
+        return 0;
+    }