This will be used when screencasting monitors so that if
there's scanout in place, it'll still be possible to blit
it to a PipeWire-owned framebuffer, and stream it.
Add a new 'blit_to_framebuffer' vfunc to CoglScanout, and
implement it in MetaDrmBufferGbm.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1421
Instead of always swapping buffers and flipping the back buffer, make it
possible to scan out a provided buffer directly without swapping any EGL
buffers.
A buffer is passed as an object implementing the empty CoglScanout
interface. It is only possible to do this in the native backend; and the
interface is implemented by MetaDrmBufferGbm. When directly scanned out,
instead of calling gbm_surface_lock_front_buffer() to get the gbm_bo and
fbid, get it directly from the MetaDrmBufferGbm, and use that to create
the page flip KMS update.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/798
