Basic biome support

Added new --biomes option, a new biomes.csv, and a new column in
textures.csv indicating whether to use the current biome's grass or
foliage colour.

Biome colours come from two .png files that are included in MC's
textures, and determined via temperature/rainfall values for each biome
(taken from MC's source code). These colours are currently hardcoded in
the spreadsheet, but could in the future be extracted from the .png
files during texture initialization.

Currently the biome colour is alpha blended onto the normal colour, and
all the hardcoded colour values are set to 50% opacity. This doesn't
look great, however - we should try converting the biome colour to HSL
and using it as a hue filter.