Add experimental support for separate HW contexts.
Add HW context support to X/EGL and KMS/EGL.
Add Windows shared context.
Build fix.
Make video_shared_context an option.

Might be part of libretro API later if necessary. Must be discussed with
other frontend developers first.
Only bind shared context if necessary.
Add async readback to GLES3.

Have support for PBOs and glMapBufferRange.
Don't create PBO scaler if not needed.
Merge pull request #654 from libretro/shared-context

libretro GL shared context support