tile.cpp, gfx.h: Make GFX.[Real]ScreenColors local variables.

GFX.ScreenColors and GFX.RealScreenColors were used to pass around the palette used for the current background's current scanlines. They were not used elsewhere. This commit makes them local variables in the tile drawing functions instead. This removes some memory stores, which are mandatory towards global variables.

Based on <https://github.com/ShadauxCat/CATSFC/commit/d1d0c81af89dc0d54e9d16a64af4a0597b4be90a> by Nebuleon.
ppu.cpp: Store products and quotients using word writes (little-endian).

Based on <https://github.com/ShadauxCat/CATSFC/commit/2572e0736928742b8d02e18786d2c914fd956335> by Nebuleon.