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.

(Ported from amdgpu commit bf326f2ea19daa6c8da23d6788ff301ae70b8e69)