This adds a third profile to the available profiles to map device-specific
speed to an acceleration factor, fully defined by the caller.

There has been a consistent call for different acceleration profiles in
libinput, but very little specifics in what actually needs to be changed.
"faster horses" and whatnot (some notable exceptions in e.g. bug 101139).
Attempts to change the actual acceleration function will likely break things
for others.

This approach opens up the profile itself to a user-specific acceleration
curve. A caller can set an acceleration curve by defining a number of points
on that curve to map input speed to an output factor. That factor is applied
to the input delta.

libinput does relatively little besides mapping the deltas to the
device-specific speed, querying the curve for that speed and applying that
factor. The curve is device-specific, the input speed is in device units/ms.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>