Pointer acceleration update

[Simon Thum]

Hi all,

between jobs I found some time to give some TLC to pointer acceleration, 
mostly it's about minor issues and cleanup that accumulated over time.

The most interesting one is the removal of the "lightweight" scheme and 
some of scheme switching, and the fix to profile 4 (which is in effect 
by default). The fixes to softening should restore lost subpixel precision.

To make sure that it's not just one well-tested profile which is faulty, 
I also added a unit test for the profiles whose output can be plotted 
with gnuplot. So if you are really curious, you can adjust the test 
parameters, e.g. to those you use, and have the profiles plotted. If you 
are less curious, look at the graphs I attached (I hope it's OK to spend 
160k on this).

The graphs use the defaults (acceleration = 2.0; threshold = 4.0) with 
adaptive deceleration = 1 (default) and 3. The "buggy" graph just 
displays the faulty previous profile 4 for reference.

Aside from the unit test, I admittedly have not given them too much 
testing, but they're mostly trivial except for number 5 (lightweight 
scheme removal) which is independent from the rest. I will use them 
myself as soon as an unrelated xcb problem is fixed, but in general I 
think they are safe.

Safe, that is, except for the API and ABI changes produced by the 
switching removal. If desired an equivalent without ABI breaks can be 
made, but will leave unused stuff around. Since a release has just been 
issued I opted for the break.

Cheers,

Simon
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