This aims to make future refactoring easier. The arguments are removed where possible, but kept and marked with C++17's [[maybe_unused]] where they could not be removed (e.g. when they are used as a callback, rather than called directly).
I've skipped the rides/<category>/* and peep/* source files, because the rides source files are mostly generated and have a ton of unused variables, and the peep source files are being refactored.
I've also skipped most of window/* source files, because most of the functions are used as callbacks and will be bulk-renamed at some point.
The game will now check for the number of entries in CSG1.DAT. If it's too few, it will now no longer try using its sprites, but print out a warning instead.
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.
