Add more specific tests for decoding

[ScanMountGoat]

This can be done using encoded images either as DDS files or raw bytes and their corresponding decoded image. Images should test the min and max values for endpoints as well as values in between to test interpolation and rounding. Blocks and RGB channels within each block are decoded independently, so this limits the number of test blocks required. It may even be possible to "exhaustively" test the individual RGB channels for the smaller formats. The challenge will be finding a suitable reference decoder since only bc6 and bc7 are required to be bit accurate.
https://fgiesen.wordpress.com/2021/10/04/gpu-bcn-decoding/