`tex.setPixels` binds to unit zero

[mattdesl]

This seems to be causing issues with gl-audio-analyser
stackgl/gl-audio-analyser#1

Because the bind() calls in setPixels is not necessarily going to be the same active unit as the user requests with tex.bind(n), it seems like it is causing some issues. I didn't realize this but according to the spec, the two seem tied somehow:

glTexImage2D specifies a two-dimensional or cube-map texture for the current texture unit, specified with glActiveTexture.

[hughsk]

I wonder if we should keep track of the most recently bound texture unit for that instance, and bind to that? So:

tex1.setPixels(...) // binds to 0

tex1.bind(3)
tex2.bind(5)
tex1.setPixels(...) // binds to 3

A little less likely to cause issues, but it might still result in weirdness if you're working at the WebGL level. Either way would be worth one of us documenting this behaviour in the readme's setPixels() docs when/if we next have a chance :)