Merge pull request #32 from playcanvas/master

merge
Single mipmapped prefiltered cubemap instead of 6 by using EXT_shader_texture_lod. Only enabled when extension exists AND device has less than 16 samplers (mobiles).
gfx.device can now upload textures with multiple mips.
Fix
Merge pull request #119 from guycalledfrank/cubemap-texture-lod

Cubemap texture lod