glamor: make sure the correct FBO is cleared
This also removes an unnecesary call to glDrawBuffer. Signed-off-by: Dor Askayo <dor.askayo@gmail.com> Fixes: 0e9a0c20 - "xwayland: clear pixmaps after creation in rootless mode" Closes: xorg/xserver#933 Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>