Renamed plants.h to be txg_plants.h.

-Peter Mawhorter
Got basic stone texture generation working.

-Peter Mawhorter
Worked on stone textures.

-Peter Mawhorter
Work on the texture generation system: got stone texture generation to a
pretty good state; need to wire through stone species generation now.

-Peter Mawhorter
Work primarily on material properties of stone.

-Peter Mawhorter
More work on geology and materials. Igneous rocks now have full material
generation (although some properties like malleability may need tweaking
later). Metamorphic and sedimentary material property generation is
next, and can draw on igneous generation as a base. Texture generation
parameters will also need to be added.

-Peter Mawhorter