Fix rP25169 - Mouse wheel hotkeys behaving erratically.

Mouse wheel hotkeys behave differently from other hotkeys (mouse button/keys) in that they are transient: they're always pressed/released in one go.
As such:

• They shouldn't be added to the stored pressedHotkeys
• They shouldn't be added to active scancodes.

This fixes these issues from rP25169 & also skips un-necessary processing in those cases.

Reported by: Imarok.

Differential Revision: https://code.wildfiregames.com/D3782