Material cleanup. Closes #8 on Obsidian

- Removed CustomCrops (needs complete re-thinking)
- Removed material constants. Magma leaves Material handling up to the
  addon developer.
- Cleanup OBJ block rendering. It doesn't work 100% yet as it has texture
  problems but I am close to fixing that***.

  ***The code before "worked" but was incorrect in that it flushed
  tessellation, binded block texture, performed tessellation, flushed
  tessellation, and then rebound the stiched texture. Its a clever fix in
  theory but destroys surrounding textures of the chunk as tessellation is
  sometimes flushed mid-draw (aka missing block textures).

Signed-off-by: Chris Sanders <zidane@outlook.com>