Added DEPTH_STENCIL_ATTACHMENT and DEPTH24_STENCIL8 which were missing, but part of webGL spec.
Added DEPTH_STENCIL
image num_bytes incorrectly defined as width*height. Should be width*height*num_channels. Currently, it seems only RGBA format is used, hence hardcoded 4
Merge pull request #8 from pastasfuture/master

Added support for depth + stencil render targets as in webGL spec