TS/RA2 Voxel support

This adds everything necessary to parse vxl/hva assets, render them ingame, and traits suitable to wire up most of the TS/RA2 units. I finally have this in a state that i'm mostly happy with, so it's time to get some external feedback.

See my [voxeldemo](https://github.com/pchote/OpenRA/tree/voxeldemo) branch for a testcase which replaces C&C units with TS equivalents. It expects TIBSUN.MIX and ts_conquer.mix (extract and rename CONQUER.MIX to avoid a naming conflict) to exist in ^/Content/ts/