Pi4 & palette fixes/improvements
- Disabled gamma on Pi 4, pending resolution of https://github.com/raspberrypi/firmware/issues/1274
- Adjusted dispmanx coordinate limits & default clock limits for Pi 4
- Update GraphicsV WritePaletteEntry & WritePaletteEntries to only claim success for supported palette types
- Implement GraphicsV ReadPaletteEntry, after discovering that in 8bpp modes the palette entries are only 16 bits wide