Alpha-premult and y-flip are deprecated for non-DOM-Element uploads.

I'm getting this error in FF 67
```Error: WebGL warning: texImage2D: Alpha-premult and y-flip are deprecated for non-DOM-Element uploads.```

Caused by https://github.com/pex-gl/pex-context/blob/master/texture.js#L90 which should be moved to this if check https://github.com/pex-gl/pex-context/blob/master/texture.js#L118