Support for hardware rendering

[codebutler]

• Core calls the "environment" callback with RETRO_ENVIRONMENT_SET_HW_RENDER https://github.com/libretro/libretro-common/blob/master/include/libretro.h#L546
• Frontend receives that callback and returns a pointer to a struct retro_hw_render_callback
• That struct has two important fields that need to be populated by the frontend:
  • get_proc_address -- pointer to a function that resolves functions by name.
    • This function gets called by the core for every GL symbol. https://github.com/libretro/libretro-common/blob/master/glsm/glsm.c#L2256 https://github.com/libretro/libretro-common/blob/master/glsym/rglgen.c#L41
    • The list of symbols depends on the gl version being used: https://github.com/libretro/libretro-common/blob/master/glsym/glsym_gl.c https://github.com/libretro/libretro-common/blob/master/glsym/glsym_es2.c https://github.com/libretro/libretro-common/blob/master/glsym/glsym_es3.c
    • RetroArch has an implementation of get_proc_address for Android that simply calls thru to egl_get_proc_address.
  • get_current_framebuffer -- pointer to function that returns an opengl framebuffer
    • TODO: Figure this out.
• Struct also has a field context_reset pointing to a function that needs to be called by the frontend to initialize/reset hw rendering.