Increase default atlas size to 2048x2048

According to http://webglstats.com/#h_texsize, 100% of WebGL users
in the wild support up to 2048x2048 textures. With 4x4 repeated/padded
textures 16x16 each, this will support up to 1024 textures per atlas.

(moved from deathcap/voxel-engine@a8ef28f7c2645dc106737bbae556e816d2409fbe)
0.1.0