Restore texture api: loadTextures and applyTextures

I removed `loadTextures` and `applyTextures` in my last commit but I realized they should probably still be exposed. Rather than each app needing `voxel-texture` required to texture things. This way voxel creature [can be obsidian](https://github.com/substack/voxel-creature/blob/master/example/debris.js#L31) once again.