demos: Add a dithering demo

This adds a dither.c which provides a demo of the dithering feature.
This is based on the scale.c demo for scaling and provides a selection
of intermediate formats and dithering operators (currently, only
PIXMAN_DITHER_ORDERED_BAYER_8) to use.  Images are first blitted onto a
surface of the intermediate format with the requested dither setup, then
blitted back onto a a8r8g8b8 surface for display.