fbdev: Add an fbdev compositor backend using pixman and evdev

fbdev: Add an fbdev compositor backend using pixman and evdev

Initial version of a frame buffer backend using pixman to render to fbdev.
This has been tested against nouveaufb but nothing else. Much of the code
came straight from the rpi backend (and copyright has been attributed
accordingly).

The behaviour of this backend on less modern frame buffers has yet to be
tested.