glamor: Make pixmap scanout compatible if its dimensions are

Namely, if its dimensions match those of the screen pixmap (enough that
it could stand in for it). When that's the case, the pixmap may end up
being scanned out directly due to page flipping via the Present
extension, e.g. with xfwm4 --vblank=xpresent .

v2:
* Use AMDGPU_CREATE_PIXMAP_SCANOUT instead of second-guessing in
  amdgpu_alloc_pixmap_bo, fixes corruption when resizing from smaller
  to larger virtual size via RandR.

Closes: xorg/driver/xf86-video-amdgpu#10
12 jobs for glamor-pixmap-scanout in 1 minute and 20 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #2374091
debian-testing

00:00:21

 
  Build
passed #2374094
xserver-1.13

00:00:46

passed #2374095
xserver-1.14

00:00:49

passed #2374096
xserver-1.15

00:00:31

passed #2374097
xserver-1.16

00:00:47

passed #2374098
xserver-1.17

00:00:32

passed #2374099
xserver-1.18

00:00:46

passed #2374100
xserver-1.18-clang

00:00:51

passed #2374101
xserver-1.19

00:00:49

passed #2374102
xserver-1.19-clang

00:00:52

passed #2374092
xserver-1.20

00:00:58

passed #2374093
xserver-1.20-clang

00:00:50