Simplify command line about astc-encoder

[solidpixel]

Proposal for rationalizing HDR settings:

We now have -ch for selecting the HDR profile, but still have separate -hdr and -hdra options which are needed to correctly set up the encoder heuristics even if the profile is HDR. For HDR textures -ch by itself is really useless, and for non-HDR textures you shouldn't be using -ch in the first place.

To reduce the confusion, and make creating a library interface simpler, I propose merging these three separate flags into two mutually exclusive compressor profiles:

• Option opn: -ch (HDR RGB, with LDR alpha) and -cH (HDR RGBA).
• Option two: -ch (HDR RGB, with LDR alpha) and -cha (HDR RGBA).
• Option three: -cht (t=transparency HDR RGB, with LDR alpha) and -ch (HDR RGBA).

If we go for -cht then we could also have -clt and -cst and make it so that that implies -a 1? Not 100% sure that I like this as an option; not having alpha weighting gives some options to pack non-color with HDR RGB data.

[solidpixel]

Proposal for rationalizing alpha blending settings.

We currently have two separate settings for weighting RGB errors by the alpha channel, which ensures alpha quality is good in regions with low transparency. We have the generic -a <radius> option. We also have -alphablend which is simply an alias for -a 1. Propose removing -alphablend - it's not adding much.

[solidpixel]

Alternative for the direct vs perceptual changes would be to keep both, but separate this out from the color format. Default should be "direct", as the perceptual metrics can have problems (e.g. hdr_log is known to give blocky output)

[solidpixel]

All planned changes now merged.