This patch completely removes the Raspberry Pi backend and the renderer.

The backend and the renderer were written to use the proprietary
DispmanX API available only on the Raspberry Pi, to demonstrate what the
tiny computer is capable of graphics wise. They were also used to
demonstrate how Wayland and Weston in particular could leverage hardware
compositing capabilities that are not OpenGL. The backend was first
added in e8de35c9, in 2012.

Since then, the major point has been proven. Over time, support for the
rpi-backend diminished, it started to deteriorate and hinder Weston
development. On May 11, I tried to ask if anyone actually cared about
the rpi-backend, but did not get any votes for keeping it:
https://lists.freedesktop.org/archives/wayland-devel/2016-May/028764.html

The rpi-backend is a good example of how using an API that is only
available for specific hardware, even more so as it is only available
with a proprietary driver stack, is not maintainable in the long run.
Most developers working on Weston either just cannot, or cannot bother
to test things also on the RPi. Breakage creeps in without anyone
noticing. If someone actually notices it, fixing it will require a very
specific environment to be able to test. Also the quality of the
proprietary implementation fluctuated. There are reports that RPi
firmware updates randomly broke Weston, and that nowadays it is very
hard to find a RPi firmware version that you could expect to work with
Weston if Weston itself was not broken. We are not even sure what is
broken nowadays.

This removal does not leave Raspberry Pi users cold (for long), though.
There is serious work going on in implementing a FOSS driver stack for
Raspberry Pi, including modern kernel DRM drivers and Mesa drivers. It
might not be fully there yet, but the plan is to be able to use the
standard DRM-backend of Weston on the RPis. See:
http://dri.freedesktop.org/wiki/VC4/



The rpi-backend had its moments. Now, it needs to go. Good riddance!

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Acked-by: Bryce Harrington <bryce@osg.samsung.com>
Acked-by: Jonas Ådahl <jadahl@gmail.com>
Acked-by: Daniel Stone <daniels@collabora.com>