add libretro platform

[azaka]

Allow the project to be built as a libretro core.

Currently video and input with joystick are tested to work.

Uses software renderer from https://github.com/Un1q32/Butterscotch/tree/software

[Un1q32]

PRs that require the software renderer should be marked as draft and wait until #128 is merged.

[azaka]

Audio added. Miniaudio is used as mixing engine

[leap0x7b]

why software renderer? couldn't opengl be used instead?

[Eliandro4]

Maybe its cuz the dev wants the core to run on a device without gou

[RicardoFromPortugal]

yeah basically software rendering first, other hw accelerated renderers can be added later down the line

[cobaltgit]

Yeah I do think the hardware renderers should be integrated into the core aswell, I (as well as the dev) work with handheld gaming devices that don't have 3D GPUs so it makes sense to integrate the software renderer though

[Un1q32]

If you added OpenGL support it could be merged sooner then if it starts as software only, since it wouldn't need the sw renderer to be useful.

[azaka]

Core is now hardware accelerated through gles3. Building with software rendering is optional.