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.

!798