Firefox shows warnings of CPU side conversions for images

The following warning appears per image.
```
Error: WebGL: texImage2D: Incurred CPU-side conversion, which is very slow.
Error: WebGL: texImage2D: Incurred CPU pixel conversion, which is very slow.
Error: WebGL: texImage2D: Chosen format/type incurred an expensive reformat: 0x1908/0x1401
```