Regression from #6574. Restoring the backup of SPR_TEMP was removed which was necessary as it is the sprite for grass tile zoomed out. Instead make SPR_TEMP a special sprite ID which is not used for anything else apart from temporary sprite drawing.
Refactor gUnk9DE568 and gUnk9DE56C to be type of rct_xy16 called gPaintSpritePosition
Rename unk_9ABDA4 to gCurrentColourPalette.
Rename gUnkEDF81C to gCurrentImageType.
Rename sub_679236_679662_679B0D_679FF1 to pixel_is_present.
Remove unused variables _unk9ABDAE, _unk9AC149, unk_9E3CE4, only ever assigned but never read.
Remove unreferenced variable unk_9E3CDC.
Clean up some parts in sub_679236_679662_679B0D_679FF1.
Remove function sub_68371D and references, it assigns variables that are never used.
Add assert determine if unused code.