This just creates a 1x1 RGBA texture and then reads it back in
COGL_PIXEL_FORMAT_A_8 format. Gnome Shell is doing this to create a
shadow and I accidentally broke it so this should hopefully stop that
happening again.
https://bugzilla.gnome.org/show_bug.cgi?id=671016
Reviewed-by: Robert Bragg <robert@linux.intel.com>