Fix unused parameter warnings

Make -Wall -Wextra a bit more useful, by hiding unused parameter warnings at every place they're unused for valid reasons. 

Except "Mark unused coord/element parameters in UI functions." commit, where I didn't analyze thoroughly why they were unused, but just assumed that if there were rendering bugs they'd have manifested already. If this commit would be deemed dubious, I can resubmit this PR without it, though it fixes almost a half of the warnings.

There are unused parameter warnings left after this commit, but they either indicate places of valid concern, like in edit_selection() function, or it's unapparent if hiding them is justified.