Starting with CMake 3.7, we can avoid dealing with individual variables
for include dirs, libraries, ... for dependencies found using
pkg-config, and instead just reference them using an imported target.
Rather than using fixed values for all biomes, generate grass and foliage
colors based on (height-dependent) temperature and rainfall parameters.
This changes a lot of colors; in general, the new colors should be closer
to the actual Minecraft rendering. It also fixes a few colors that were
completely off (like in the badlands biomes) and adds the MC 1.13 biomes
(which were rendered in black).
Some of the newer biomes (warm/cold/... oceans) don't have proper
parameters yet, and we do not derive the water color from the biome yet.
Fixes#1.