glamor: Avoid glamor_create_pixmap for pixmaps backing windows

If the compositing manager uses direct rendering (as is usually the case
these days), the storage of a pixmap allocated by glamor_create_pixmap
needs to be reallocated for sharing it with the compositing manager.
Instead, allocate pixmap storage which can be shared directly.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
12 jobs for dri3_fd_from_pixmap-flush in 1 minute and 14 seconds (queued for 1 second)