Remove support for PVRTC texture encoding and decoding

[Calinou]

On the only platform where PVRTC is supported (iOS), ETC2 generally supersedes PVRTC in every possible way. The increased memory usage is not really a problem thanks to modern iOS' devices processing power being higher than its Android counterparts.

This closes godotengine/godot-proposals#3688 and closes #50787.

[fire]

I am ok with this. Not sure how to ping the wider community.

[akien-mga]

I think we can go ahead with this, just needs a rebase.

A couple more occurrences to remove:

$ rg -i pvrtc -g'!*.po' -g'!*.pot' -g'!thirdparty/*/'
COPYRIGHT.txt
381:Files: ./thirdparty/pvrtccompressor/
382:Comment: PvrTcCompressor

doc/classes/RenderingServer.xml
1310:                           Returns [code]true[/code] if the OS supports a certain feature. Features might be [code]s3tc[/code], [code]etc[/code], [code]etc2[/code] and [code]pvrtc[/code].

doc/classes/Image.xml
138:                            [b]Note:[/b] The following formats can be decompressed: DXT, RGTC, BPTC, PVRTC1. The formats ETC1 and ETC2 are not supported.

modules/basis_universal/SCsub
28:    "basisu_pvrtc1_4.cpp",

thirdparty/README.md
539:## pvrtccompressor
541:- Upstream: https://bitbucket.org/jthlim/pvrtccompressor (dead link)
542:  Unofficial backup fork: https://github.com/LibreGamesArchive/PVRTCCompressor

And well the pvrtccompressor library itself.

[bruvzg]

Should be fine.

For Metal, ETC2 should be supported by all iOS devices with PVRTC support - https://developer.apple.com/metal/Metal-Feature-Set-Tables.pdf

For OpenGL ES 3.0, 2013+ iOS devices also have ETC2 support, in the old docs PVRTC textures are recommended (without explanation why).

[akien-mga]

As discussed on Rocket.Chat, thirdparty/pvrtccompressor should be removed, as well as references to it in thirdparty/README.txt and COPYRIGHT.txt.

[akien-mga]

Thanks!