minor cleanup

use hypotf() instead of sqrt()
Fix missing brushdabs with "slow_tracking_per_dab"

Fix a glitch where individual brush dabs are skipped if the
"slow_tracking_per_dab" setting is used.

https://github.com/mypaint/mypaint/issues/2

The glitch was visible best with brushes that make smooth strokes that are
not fully opaque.

The filter for "slow_tracking_per_dab" was always updating as if the
brush had been moved forward by a full dab. However the code is also
called to move the brush forward by less than one dab. As a result the
filter did catch up the brush position too fast at motion events.

This changes existing brushes slightly. For the brushes I tested the
change was either a subtle improvement or not perceivable at all.
It seems unlikely that a brush relied on this bug for getting roughness.

List of MyPaint stock brushes using "slow_tracking_per_dab":

classic/brush.myb
classic/calligraphy.myb
classic/ink_eraser.myb
classic/pen.myb
classic/slow_ink.myb
deevad/2B_pencil.myb
deevad/fill.myb
deevad/large_hard_eraser.myb
deevad/pen.myb
deevad/pen-note.myb
deevad/rough.myb
deevad/thin_hard_eraser.myb
experimental/glow.myb
experimental/irregular_ink.myb
experimental/subtle_pencil.myb
ramon/100%_Opaque.myb
ramon/Delayed_.myb
ramon/Hard_Eraser.myb
ramon/Marker.myb
ramon/PenBrush.myb
rename some variables, update docu

It was called "dist" but the distance is actually measured in dabs, so
call it "dabs" now. Update PNG accordingly.