Adding width/height convenience properties

IMO `tex.shape[1]` is a bit tedious and counter-intuitive, and width/height properties would be good. Not sure about setters, since it doesn't actually change the GPU texture, so maybe they should be disabled?

I'm starting to pull kami modules out of core, and everything is playing pretty nicely with gl-texture2d so far. This change would allow the kami batcher to fall back to texture size when the user doesn't specify it explicitly.

https://github.com/mattdesl/kami-batch/blob/master/examples/modules-gl.js