Fixed bug 4797 - SDL fails to compile with Mesa Master (thanks Michael Olbrich!)

fix building with Mesa 19.2

With Mesa 19.2 building fails with:

/include/GLES/gl.h:63:25: error: conflicting types for 'GLsizeiptr'

The same type is defined in include/SDL_opengl.h for OpenGL and the two
headers should not be included at the same time.
This was just never noticed because the same header guard '__gl_h_' was
used. This was changed in Mesa. The result is this error.

Fix this the same way GLES2 already handles this: Don't include the GLES
header when the OpenGL header was already included.
(https://hg.libsdl.org/SDL/rev/6a3670d6108d)

src/video/SDL_video.c

@@ -37,9 +37,9 @@
 #include "SDL_opengl.h"
 #endif /* SDL_VIDEO_OPENGL */
 
-#if SDL_VIDEO_OPENGL_ES
+#if SDL_VIDEO_OPENGL_ES && !SDL_VIDEO_OPENGL
 #include "SDL_opengles.h"
-#endif /* SDL_VIDEO_OPENGL_ES */
+#endif /* SDL_VIDEO_OPENGL_ES && !SDL_VIDEO_OPENGL */
 
 /* GL and GLES2 headers conflict on Linux 32 bits */
 #if SDL_VIDEO_OPENGL_ES2 && !SDL_VIDEO_OPENGL