Add libretro port

[iyzsong]

Changes:

Add libretro port, which build VVVVVV as a shared library (static link not tested) for RetroArch.
It:

• Add SDL2 as a submodule to third_party
• Use SDL software renderer
• Add BUILD_LIBRETO and LIBRETO_STATIC options to cmake
• Will build vvvvvv_libretro.so on Linux, run via retroarch -L vvvvvv_libretro.so.
• game data (data.zip, lang and fonts) should be in a vvvvvv directory in the RetroArch system directory.

To keep it smooth, enable '>30 fps' and disable 'vsync'.

Thanks!

Legal Stuff:

By submitting this pull request, I confirm that...

• My changes may be used in a future commercial release of VVVVVV
• I will be credited in a CONTRIBUTORS file and the "GitHub Friends"
  section of the credits for all of said releases, but will NOT be compensated
  for these changes unless there is a prior written agreement

[NyakoFox]

How does this work with custom levels with custom assets? Isn't there a problem regarding save state size being dynamic due to custom assets size being dynamic?

[iyzsong]

How does this work with custom levels with custom assets? Isn't there a problem regarding save state size being dynamic due to custom assets size being dynamic?

I haven't test it, but it can load data.zip as game, and use its directory to replace the system directory. The saves are not managed by RetroArch, and will be put in the system directory's saves.

[InfoTeddy]

How does this work with custom levels with custom assets? Isn't there a problem regarding save state size being dynamic due to custom assets size being dynamic?

This doesn't support save states (retro_serialize and retro_unserialize return false).

[flibitijibibo]

The changes to the core are okay, but the added SDL files are kind of huge - we did a bunch of changes to SDL's overall design to accommodate RetroArch's expectations for program flow, has a libretro port of SDL3 been attempted yet? The custom files actually look okay so I wonder if now's the time to upstream, and then we can get this port in when #1200 is done.