libretro: update frameskip logic

libretro: update frameskip logic

As PSX usually runs at 20fps or so, underruns were not noticed because
the following frame after underrun libretro would indicate none, but the
core's logic would only evaluate it when the game flips the buffers,
which is like every 3rd frame or so. With that frameskip on slow devices
was pretty much ineffective.