glBlitFramebuffer is affected by the scissor so we need to ensure
there is an empty clip flushed before using it. This is similar to
what is done in _cogl_blit_framebuffer().
https://bugzilla.gnome.org/show_bug.cgi?id=690451
Reviewed-by: Robert Bragg <robert@linux.intel.com>
Tested-by: Aaron Plattner <aplattner@nvidia.com>
(cherry picked from commit 65da3f88af9c7b8d72758d116c2652aff68195c1)
f1baa4d60f