st/xorg: Add support for dirty framebuffer region reporting.

st/xorg: Add support for dirty framebuffer region reporting.

Add a BlockHandler which flushes the context and reports the dirty region
gathered using the X server damage layer.

In the interim, with dirty region reporting only allocate textures for the
framebuffer and shared pixmaps (e.g. DRI2 buffers) and fall back to software
for other pixmaps. This will be improved in the future.