Non power of 4 BC1 texture fallback code can lead to a crash with some textures

Observed in VRChat with a 2019-built 2x2 BC1 texture. The fallback code truncates the number of mips from 3 to 2 which puts the texture at a size (20B) which then leads to a some memory getting corrupted which causes a crash on free.

Likely reproducable with a 2x2 BC1 texure with 3 mips being loaded into a 2022.3 player and hitting the fallback code. 

We haven't been able to manufacture a standalone repro project for this, although the pre-build bundle that