LIBRETRO: add libretro platform

[spleen1981]

No description provided.

[BallM4788]

As someone who's unfamiliar with how libretro works (and with only passing knowledge of RetroArch), what's the difference between this PR and your work over at libretro/scummvm? Does that repository not already provide what's needed to make a ScummVM core for RetroArch?

I'm not a member of the ScummVM team, but a PR description may help in case team members have that question as well.

[spleen1981]

There is no difference with current libretro core. It's same sources rearranged to fit ScummVM tree (while current core sources are structured as a wrapper with a scummvm git submodule). Purpose is to eventually drop current core repo and directly use ScummVM official repo instead.

This has been discussed with ScummVM Team in advance.

[i30817]

Hopefully the code here keeps being worked on until at least the opengl renderer works, not just keep building. That was always the worst loss on libretro-scummvm, especially for 3d games. Although i guess that's 'difficult' if you want it to happen on the console implementations of opengl.

[sev-]

Mostly looks good. I had only a few notes/suggestions

[sev-]

I have no further notes so far.