AVIF more options

[foxyshadis]

There are a handful of things missing from the AVIF settings panel that might be useful, for those who might want to use them:

* Lossless: Since the format supports it anyway.
* Auto/8/10-bit output, instead of just 8: Even with 8-bit input, forced 10 bit output is sometimes dramatically better quality, especially at lower sizes.
(Granted, I'd love an entire 16-bit pipeline in xnview someday, but that's a distant dream.)
* AQ-mode 1: Sometimes dramatically better quality at any size, but usually at least some perceptible improvement in flat or dark areas.

[foxyshadis]

I'd like to bump this, as they're mostly just checkboxes that lead to an extra parameter internally.

And one other thing:
* Allow RGB as an output colorspace, to guarantee fully lossless end-to-end, otherwise the conversion to/from YUV introduces some errors even at 4:4:4.

I guess I accidentally posted to the wrong forum, could this be moved to feature requests as well?