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>
3 jobs for remove-unnecessary-drawbuffer-call in 7 minutes and 22 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Docker Image
passed #1004125
debian-testing

00:00:38

 
  Build And Test
passed #1004126
autotools-build-and-test

00:06:44

passed #1004127
meson-build-and-test

00:01:43