Render: Allow empty cliprect.

This fixes an issue where an empty cliprect is treated the same as a NULL
cliprect, causing the render backends to disable clipping.

Also adds a new API, SDL_RenderIsClipEnabled(render) that allows you to
differentiate between:
 - SDL_RenderSetClipRect(render, NULL)
 - SDL_Rect r = {0,0,0,0}; SDL_RenderSetClipRect(render, &r);

Fixes https://bugzilla.libsdl.org/show_bug.cgi?id=2504

src/dynapi/SDL_dynapi_overrides.h

@@ -575,3 +575,4 @@
 #define SDL_GetDefaultAssertionHandler SDL_GetDefaultAssertionHandler_REAL
 #define SDL_GetAssertionHandler SDL_GetAssertionHandler_REAL
 #define SDL_DXGIGetOutputInfo SDL_DXGIGetOutputInfo_REAL
+#define SDL_RenderIsClipEnabled SDL_RenderIsClipEnabled_REAL

src/dynapi/SDL_dynapi_procs.h

@@ -606,3 +606,4 @@ SDL_DYNAPI_PROC(SDL_AssertionHandler,SDL_GetAssertionHandler,(void **a),(a),retu
 #ifdef __WIN32__
 SDL_DYNAPI_PROC(SDL_bool,SDL_DXGIGetOutputInfo,(int a,int *b, int *c),(a,b,c),return)
 #endif
+SDL_DYNAPI_PROC(SDL_bool,SDL_RenderIsClipEnabled,(SDL_Renderer *a),(a),return)