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)
3b0dcaf474