Use TS shroud sprites.

Part 2 / 5 of the isometric terrain implementation.  This generalizes our shroud rendering to support TS's quirks (shared palette, but two different sets of sprites).

The wavy lines are caused by a combination of #4718 and the isometric -> rectangular rendering.  The tiling is improved when rendered in the diamond grid (see [pchote:isometric](https://github.com/pchote/OpenRA/commits/isometric)).