Add terrain priority rendering mode in Atlas.
Fix terrain painting in Atlas to be less profligate with priority
values.
Support hotloading of blend textures.
Clean up some trivial things.
This was SVN commit r8872.
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.
