Replace almost all texture uses with calls to the new system.
Add some anistropic filtering to terrain textures.
Let Atlas load terrain texture previews partly-asynchronously by
polling.
Fix inefficient texture colour determination for minimap.
Remove unused global g_TerrainModified.
Change GUI texcoord computation to be less efficient but to cope with
dynamic texture changes.
Fix GUI renderer effects leaving bogus colour state.
This was SVN commit r8099.
