Revert to pre-atlas voxel-texture. Fixes GH-14

voxel-texture 0.5.0 switched to use texture atlases, packing all the block
textures into one graphic for efficiency purposes. However, this introduces
two problems:

- With the culled mesher (default as of voxel-engine 0.17+), I get serious lag
spikes (30-60 ms frames, or unplayably higher) when placing/breaking blocks, and also slow
page load as the meshes are built (see GH-10).

- With the greedy mesher, performance is much better (fixes GH-10) but textures are stretched
across each of the greedily constructed meshes, instead of tiled across each block (see GH-14).
Example: https://f.cloud.github.com/assets/5897956/1651233/1f478c9e-5aca-11e3-82cc-4028f1e1b1da.png

There are techniques for texturing greedy meshes, detailed in this article:

http://0fps.wordpress.com/2013/07/09/texture-atlases-wrapping-and-mip-mapping/

- Using array textures is ideal, but they won't be supported until WebGL 2.0 at earliest
https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7

- Otherwise, they can be tiled manually, but this requires a few tricks, would certainly
be worthwhile to implement, but is fairly complex. Maybe later, once I understand it better.

So for now, (unfortunately) I've reverted back to voxel-texture without atlases. This keeps
mesh recalculation fast (when breaking/placing blocks) and avoids stretched textures but
may slow down complex scenes with many materials (see performance improvement @shama noted in https://github.com/maxogden/voxel-engine/pull/69).