[solidpixel]

This PR is a collection of small bug fixes.

• Correct HDR endpoint encoding to not overflow fields after rounding.
• Correct HDR RGBS endpoint error weighting to not weight by color weight twice.
• Correct use of HDR -perceptual and -alphascale at the same time.
• Correct DDS format validation bugs that would cause valid DDS images to fail to be loaded.
• Correct KTX row padding handling bugs in both load and store paths.
• Clamp alpha range above zero in variance tables, to match the image loader.
• Add API rejection of a NaN quality value.
• Add API rejection of volumetric images not using volumetric block sizes, before would just compress plane 0.