Objective

• avoid several internal vec copies while collecting all the level data in ktx2 load
• merge another little piece of KTX2 Updates: ETC1s/BasisLZ, ASTC HDR, and faster Zstd #18411 (benchmarks there found this to be a significant win)

Solution

• reserve and extend

Testing

• ran a few examples that load ktx2 images, like ssr. looks fine

Future work

• fast path logic to skip the reading into different vecs and just read it all in one go into the final buffer instead
• as above, but directly into gpu staging buffer perhaps