Add GitHub Actions CI

[Margen67]

AppVeyor is slow (only one job at a time) and requires additional setup (registering an account, adding the repository, etc) while GitHub Actions doesn't.

Example run: https://github.com/Margen67/mgba/actions/runs/5160558114
Linux and macOS seemingly build fine.

Windows is a different story:

• Docker:
  mkdir: cannot create directory 'build-win64': Permission denied
  https://github.com/Margen67/mgba/actions/runs/5161053167/jobs/9297716540#step:3:86
  Running docker with sudo doesn't fix this.
  If I create the build-win64 folder beforehand it spews a bunch of other strange errors, possibly due to not having write permission:
  https://github.com/Margen67/mgba/actions/runs/5160690280/jobs/9296967058#step:4:85
  I'm not sure if the other Docker images are broken.
• vcpkg:
  Windows requires epoxy module!
  Without -DZLIB_LIBRARY=zlib:
  • Could NOT find ZLIB (missing: ZLIB_LIBRARY) (found version "1.2.11")
• msys2:
  • Works*
    Slow to setup/build, and not very useful since it requires tons of DLLs which cpack doesn't include in it's archive.
• Cross-compiling from Linux:

  Failed to find "GL/gl.h" in "/usr/include/libdrm".
  

  Installing libglu1-mesa-dev and mesa-common-dev doesn't fix this.

Hopefully someone who knows what they're doing can help :^)

[endrift]

This sure doesn't look ready to merge. Lots of comments and stuff broken still.

Further, I'm not really sure what the point of this is when mGBA already has lots of CI. For forks maybe?

[Margen67]

This sure doesn't look ready to merge. Lots of comments and stuff broken still.

Further, I'm not really sure what the point of this is when mGBA already has lots of CI. For forks maybe?

It's mainly for forks, yes.