Add support for PVRZ files

[lynxlynxlynx]

It's a format in use in EEs to avoid the limits of global image palettes. It would be useful already for our demo, since it's really annoying to have to deal with BAMv1 and degraded assets. Once PVRZ is supported, BAMv2 is trivial to implement, since it just uses the new data format, the rest is the same.

So far the data is using only DXT1 and DXT5 compression formats.

Useful links:
PVRZ on IESDP, discussions: PVR and PVRZ formats
And from Beamdog themselves in go (pvr, dxt): https://github.com/Beamdog/bgfileformats

[lynxlynxlynx]

The official docs are a hard to navigate maze and kinda useless:
https://github.com/powervr-graphics/Native_SDK
https://powervr-graphics.github.io/Documentation/SDKBrowser/Framework.html

Finally I did manage to find existing clean implementations for the texture decompression, just no standalone p'n'p libraries:
https://github.com/Benjamin-Dobell/s3tc-dxt-decompression (old, base for ffmpeg)
https://github.com/SoLaRGit/openS3TC (optimized + a few fixes)
https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/texturedsp.c (ffmpeg)

Doesn't look like ffmpeg supports whole pvr(z) files, but the two forum posts describe them well enough. We should just lift one implementation of decompression and write the little boilerplate ourselves.

edit: another implementation and list of resources: https://github.com/Sampsca/PS-BAM/blob/master/lib/PS_PVR.ahk
and another one, also for IE specifically: https://github.com/InfinityTools/go-ietools/blob/master/pvrz/pvrz.go
... which is using c++ libsquish for dxt decompression https://sourceforge.net/projects/libsquish/