We haven't had a proper implementation for COMPRESS_PVRTC2 (which is PVRTC1 2-bpp) in years,
so let's drop it instead of keeping a compress type which doesn't work.
The other enum values were renamed to clarify that our PVRTC2 and PVRTC4 are respectively
PVRTC1 2-bpp and PVRTC1 4-bpp. PVRTC2 2-bpp and 4-bpp are not implemented yet.
The code we had for PVRTexTool doesn't work as it's not compatible with current
PVRTexTool CLI options, and likely hasn't been for years.
Instead, we have our own vendored pvrtccompressor thirdparty library which all
users have thus de-facto been using. This commit moves the compress code to
`modules/pvr` where it belongs.
There's no proper compress function for PVRTC 2-bit format, that's a bug that
will need to be fixed (currently it's compressed as 4-bit format even if you
use Image::FORMAT_PVRTC2).
Fixes#28669.
