RetroArch: minor improvements

-GPU screenshots are possible now:
https://github.com/libretro/RetroArch/commit/28d1e77a36884e0ac036f73132f116fe56ad8e55
-PSX: Extreme slowdown if rewind is enabled. If there is no keybinding, rewind should be always disabled.
Merge pull request #282 from gizmo98/patch-1

RetroArch: minor improvements