EXA, driver allocated pixmaps and fallbacks

[Maarten Maathuis]

Imagine having tiled pixmaps, that get copied on every map/unmap.

EXA has certain optimisations to avoid migration ping-pong, ofcource
that only applies to drivers using exa's built in memory manager.

My impression is that an optional component, much like the migration
logic in exa, should be added. With one major difference, no
continuous offscreen memory.

This should reduce the penalty on sw rendering, and bring performance
back to previous levels.

In case i get bored at some point and start coding, i would like to
know if there are mistakes that should be avoided this time.

Maarten.