Rewrite shroud renderer.

This enables the original shroud artwork in C&C and D2K, and makes the shroud edges a bit smoother by using all four transition levels and varying the opacity instead of splitting into two and two. It should be forward compatible with isometric terrain for TS.

Performance is slightly faster at updating the shroud state, but potentially slightly slower at rendering (it now draws all visible tiles instead of aggregating shrouded rows).

Fixes #2162, #3024, #4034.

![shroud_cnc](https://f.cloud.github.com/assets/167819/1722147/14382fe8-623b-11e3-9206-9b0dcea245fc.png)
![shroud_d2k](https://f.cloud.github.com/assets/167819/1722148/143aa106-623b-11e3-9223-13250688d7b6.png)