Unimplemented PBGRA128F -> PBGRA32 conversion transform silently does wrong thing

[rickbrew]

As per https://discord.com/channels/143867839282020352/312132327348240384/1240119502553743461

Attempting a conversion transform from PBGRA128F Linear to PBGRA32 Companded does not work correctly. It results in blown out colors.

This is in the "internals AS-IS" area, but having an exception here because of an unimplemented/unsupported transform would be good to have. Otherwise it's hard to know when I've stepped out of bounds.

I'm not blocked; I can still use MagicScaler for the hard part of the transform (linear -> companded) and then truncate from FP32 to UI8 myself.