Add support for colourspace conversion from !=8 bit rgb

[bradh]

Currently it appears that conversion from (say) RGB is ok for 8bit input and HDR. However it is not supported for other bit depths, such as 7 bit or 5-6-5. That occurs in the uncompressed format. So reading in is OK and we do support pixel images like that. However it won't be converted to PNG or JPEG.

Ideally the colour conversion machinery would handle this. I don't know how best to do that though.

Another option would be to shift those values to 8 bit inside the codec.

[farindk]

That should be easy. We just need a color transform operation that extends <8 bit to 8bit. If we have an example image that decodes to 5-6-5, I can write that. It just might be that there are many places in the code that assume that everything that is not >8 bit is 8 bit.

[bradh]

I put in a PR at #1079 that is my first attempt at doing this. As noted in the PR, that gets me JPEG output, but not PNG (interlaced) output.

[bigcat88]

there are many places in the code that assume that everything that is not >8 bit is 8 bit.

yes, and in other libraries that use libheif there are many places where similar things are assumed, if bit != 8, then this is some kind of HDR (10,12,16 bits)

[farindk]

At least that could be shielded by the library by always converting <8 bit to 8 bit except if disabled by a decoding flag.

[bradh]

I think this is well progressed, and will close this.