xwayland: clear pixmaps after creation in rootless mode

When a pixmap is created with a backing FBO, the FBO should be cleared
to avoid rendering uninitialized memory. This could happen when the
pixmap is rendered without being filled in its entirety.

One example is when a top-level window without a background is
resized. The pixmap would be reallocated to prepare for more pixels,
but uninitialized memory would be rendered in the resize offset until
the client sends a frame that fills these additional pixels.

Another example is when a new top-level window is created without a
background. Uninitialized memory would be rendered after the pixmap is
allocated and before the client sends its first frame.

This issue is only apparent in OpenGL implementations that don't zero
the VRAM of allocated buffers by default, such as RadeonSI.

Signed-off-by: Dor Askayo <dor.askayo@gmail.com>
Closes: xorg/xserver#636
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
3 jobs for clear_pixmap in 10 minutes and 23 seconds (queued for 10 seconds)
Status Job ID Name Coverage
  Docker Image
passed #902130
debian-testing

00:01:07

 
  Build And Test
passed #902131
autotools-build-and-test

00:09:15

passed #902132
meson-build-and-test

00:01:44