fxemu.cpp: Do not use GSU.pfRpix and GSU.pfPlot to pass SuperFX plotter functions around.

Based on https://github.com/libretro/snes9x-next/commit/e9c304af88fc5f69104eee9e19e6be0a9e3e9d6e by TwinAphex51224; some additional code modifications are removed.
sa1.h, etc.: Change the type of SA1Registers to SRegisters.
All files: Replace memmove with memcpy when it can be proven that the source and destination cannot overlap.

memcpy is faster than memmove in many implementations because memcpy is specified to skip checking for overlapping buffers.

Based on https://github.com/ShadauxCat/CATSFC/commit/f385752705de73b04cbbda735a71f14c19e241a6 by Nebuleon.