gallium: Enable MESA_framebuffer_flip_y

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>

src/mesa/state_tracker/st_extensions.c

@@ -1024,6 +1024,7 @@ void st_init_extensions(struct pipe_screen *screen,
    extensions->ATI_fragment_shader = GL_TRUE;
    extensions->ATI_texture_env_combine3 = GL_TRUE;
 
+   extensions->MESA_framebuffer_flip_y = GL_TRUE;
    extensions->MESA_pack_invert = GL_TRUE;
 
    extensions->NV_fog_distance = GL_TRUE;