games/libretro-shaders-slang: udpate g20231023 → g20240719

Changes to the port:

• take maintainership
• update COMMENT and pkg-descr to point out that the shaders originate from the libretro project but that they can also be used for other projects, e.g. librashader
• update LICENSE in accordance to https://github.com/libretro/slang-shaders/issues/150 (it's actual unclear which shader has which license for the most part)
• add DOCS option which handles README.md
• update files/patch-Makefile to also delete Makefile.orig, or else it will be listed in pkg-plist

Comparing changes:
https://github.com/libretro/slang-shaders/compare/3c28137..7d6751a

PR: 280439