Partial Background Extended Palettes

[ImPerfection]

I am consistently using extended palettes for my backgrounds on the main screen, but I continue to encounter that libnds doesn't have a method of selecting the high half of the palettes, or setting the offset of the register with the extended palette selection. I have to go in there and modify the enumeration every update so that I can continue to use this functionality.

Even for those that do not want to use just one bank by itself may want to use VRAM banks F and G together for their extended palette. For that as well, the addition of the offsets into their respective enumerations would be very useful.