As proof-of-principle add the nearly working demonstrations of using DRM
to render directly with the GPU bypassing both RENDER and GL for
performance whilst preserving high quality rendering.

The basis behind developing these chip specific backends is that this is
the idealised interface that we desire for this chips, and so a target
for cairo-gl as we continue to develop both it and our GL stack.

Note that this backends do not yet fully pass the test suite, so only
use if you are brave and willing to help develop them further.