pixman-renderer: Add repaint debugging feature

Add 'R' as a debug shortcut that highlights repainted regions.
pixman-renderer: Move shadow buffer into renderer

The X11 backend uses a shadow buffer to be able to support transformed
outputs. However, this belongs in the renderer, since otherwise this
code would have to be copied into every backend that uses the pixman
renderer and supports transformed outputs.
compositor-drm: Split kms initialization out of init_egl()
compositor-drm: Delay egl initialization until all outputs are created
compositor-drm: Put logic of destroy/releasing bo's in its own function

We have this duplicated in a few places.
compositor-drm: Support creating drm_fb backed by dumb buffers
compositor-drm: Add support for using the pixman renderer

If --use-pixman is passed as command line option to weston, the drm
backend will use the pixman renderer instead of the gl one.
compositor-drm: Do SetCrtc() if flipping buffers with different stride

Page flipping to a buffer with stride different than the current one
is not possible with the intel kms driver. Work around this by doing
a mode set. This should only happen when going from dumb buffers to
egl, so the overhead shouldn't be a problem.
compositor-drm: Show a splash screen while loading gl stuff