add support for loading PVR images

[erysdren]

this format is from the Dreamcast Q3A port

documentation used:

• https://fabiensanglard.net/Mykaruga/tools/segaPVRFormat.txt
• https://github.com/buhman/dreamcast/blob/main/gen/k_means/python/decode_pvrt.py

[timangus]

I'm not sure I see the actual general use case for this tbh, is there much demand for being able to load such images outside of very specific platforms (which ioq3 doesn't currently support)? On the other hand I can't see how it would be particularly harmful to add this functionality either so...