games/libretro-shaders-slang: update g20240719 → g20240804

Comparing changes:
https://github.com/libretro/slang-shaders/compare/7d6751a..b327343

Port changes:

• remove PORTDOCS from Makefile, keep it listed in pkg-plist instead
• set DATADIR to its actual non-standard directory
• remove patch, install with COPYTREE_SHARE instead
• use the more commonly used post-install-DOCS-on target again
• pet portclippy, portfmt, portlint

PR: 280700