Replace the GL renderer with the new rpi-renderer on the Raspberry Pi
backend. This makes Weston on rpi not use EGL or GL anymore, at all.

The weston_plane feature is disabled, since the rpi-renderer does the
same, but better.

Add a command line option to select the output transform. It is not a
weston.ini option for now, since the rpi backend does not read the
configuration file yet. Hopefully that will be done later with some
shared code.

Add the rpi options to 'weston --help' output.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>