Terrain height filtering

[kvark]

No filtering is used at the moment. We could technically do some anisotropic filtering for the height map, even if that requires splitting the height levels into different channels and blows up the texture size. However, the terrain type is still discrete, and it produces the final color, so this is going to still look blocky.

[kvark]

This is actually related to #5. If we are to filter the terrain, we've got to turn the discreet meta flags into something continuous. Perhaps, just plain interpolation of the terrain type would work.

I believe the filtering is best done by representing the data in a single RGBA4 texture, roughly as:

• RG stores the top altitude (hi/lo bits are split between 2 channels)
• B stores both the low altitude and the gap to the surface (exact placement is still TBD)
• A stores the terrain type as well as potentially some other bit (baked-in shadow?..)

[kvark]

Experiments in #203 (and #65) show that any kind of filtering is not improving the quality.