xwayland: Disable window buffers if using glamor's eglstreams backend

Otherwise the first window you map will crash the server. None of this
should be necessary for streams in any case, which handles this
n-buffering directly through EGL.
4 jobs for xwayland-streams-regression-fix in 3 minutes and 38 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Docker Image
passed #4221046
debian-testing

00:00:23

 
  Build And Test
passed #4221047
autotools

00:03:15

passed #4221048
meson

00:01:10

passed #4221049
meson-noglamor

00:01:55