xwayland: Replace window pixmap as needed for drawing operation

We must not modify the contents of the pixmap whose wl_buffer is attached to the wl_surface.

If there's an available window buffer, we re-use that for the window pixmap. Otherwise we just allocate a new one.

This also avoids Present client hangs due to xwl_present_buffer_release not getting called for the buffer release event.

Fixes: 6779ec5b ("xwayland: Use window pixmap as a window buffer") Closes: #1633 (closed)