lazy platform dependencies lazy, drop unused GL registries

[nat3Github]

passing -fsys= skips the fetch and uses the system/sysroot headers instead.
The egl and opengl registries are removed: SDL already ships the EGL/KHR/GLES and GL headers it needs.

zig-pkg footprint: macOS/Windows 26 pkgs/379M -> 1 pkg/52M.
closishes #36

[MasonRemaley]

Nice thank you, this seems like a huge improvement to the way I had this set up. I'll review this soon.

CC @SpexGuy this may solve the problem you were having yesterday.

[MasonRemaley]

Thanks again for the PR! I made a few minor changes, will merge as soon as CI passes. If this breaks anything for you let me know.

(Note that I force pushed a rebase with main. I probably should've done a merge instead to keep the old commits reachable, if you have anything pointing at them you can either update to main post-merge, or push your branch to your fork under a different name to keep the pre-rebase commits reachable.)

I also added instructions to the README that explain how to use the system dependencies. You probably already have this all sorted out, but if not, I suspect you'll want to use --system dir to enable all system integrations in one go and get them from dir, though of course you can also set them one at a time with -fsys and --search-prefix.