16-bit texture IDs and 2048x2048 atlas. Closes GH-5

Requires voxel-shader 0.4.0+ and voxel-mesher 0.3.0+:

deathcap/voxel-shader@1edeec282017918ec46485a78096d21147780c1c
deathcap/voxel-mesher@cd154c880a9c8582464864da588da8909d4eccbe
0.4.0