Support RGBA image format

When dealing with OpenGL and web based formats such as canvas and WebGL, the byte-ordered RGBA format is becoming more and more common.

We have however only limited ability to work with it in Qt. The only current option is the QImage::rgbSwapped() command, which unfortunately only does the correct thing on little endian machine, and also doesn't store in the image that the RB colors have been swapped.

We should instead have full support for the byte-ordered RGBA format. To proper disambiguate it from ARGB which is often called RGBA32, I suggest using the name RGBA8888, which tells the format is byte-ordered and not 32bit ordered.