Fix streaks when exporting 16-bit integer or floating point channel file to png

When exporting 16-bit integer or float point channel images to png, even if forceSRGB is checked the exported image will still contain stripes, the banding produced seems to be due to the bit depth conversion, so I have separated the colour depth from the colour configuration, I'm not sure if this is a problem, after ticking Save as HDR in the export panel you can still tick the force conversion to 8 bit channels and it seems to work fine now