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This adds support for the MS Intellimouse & Intellimouse Explorer
extensions, as documented by:

https://isdaman.com/alsos/hardware/mouse/ps2interface.htm
https://www.scs.stanford.edu/10wi-cs140/pintos/specs/kbd/scancodes-12.html

Wheel state changes & extra buttons are signalled to the OS via
PointerReason_WheelChange