screen-cast/monitor-src: Bring back blitting
Now that the monitor screencast records to DMA-BUF buffers immediately (since bc2f1145) we have the opportunity to bring back the blitting technique.
Bring back blitting. This time, instead of simply failing if the blit fails, add a fallback path that does a stage paint if something goes wrong.
Embedded cursors should still be fine because hardware cursor is inhibited while embedded cursor screencasts are running.