> I did test negative scaling (inversion) which would succeed in giving
> me a headache pretty quickly.

For normal use, yes.  But there are plenty of special purposes for
which reversal is reasonable.  (As simple examples: it allows you to
compensate for the mouse being physically reversed, or for its
generating deltas that are reversed from how the software expects.)

I would be tempted to make it a full 2x2 (or 3x3, or whatever) linear
transform - in particular, such that it is possible to switch axes.
That may be overkill, though.

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