tablet: require a minimum pressure before we process pressure events

tablet: require a minimum pressure before we process pressure events

Tools default to 1% lower threshold (tip up) and 5% upper threshold (tip
down). But our distance vs pressure exclusion would reset the distance
for *any* pressure value, regardless how low that value was and how high
distance was in comparison.

A very low pressure value of less than 1% would then result in a
normalized pressure of 0, so we'd effectively just reset the distance to
zero and do nothing with the pressure. This can cause distance jumps
when the tool arbitrarily sends low pressure values while hovering as
seen in https://github.com/libsdl-org/SDL/pull/5481#issuecomment-1118969064

Commit 61bdc05fb0f84303f97daaba6ae6b49c976dbfbf from Dec 2017
  "tablet: set the tip-up pressure threshold to 1%"
was presumably to address this but no longer (?) works.

Fix this by addressing multiple issues at the same time:
- anything under that 1% threshold is now considered as zero pressure
  and any distance value is kept as-is. Once pressure reaches 1%,
  distance is always zero.
- axis normalization is now from 1% to 100% (previously: upper threshold
  to 100%). So a tip down event should always have ~4% pressure and we
  may get tablet motion events with nonzero pressure before the tip down
  event.
  From memory, this was always intended anyway since a tip event should
  require some significant pressure, maybe too high compared to e.g.
  pressure-sensitive painting
- where a tablet has an offset, add the same 1%/5% thresholds, on top of
  that offset. And keep adjusting those thresholds as we change the
  offset. Assuming that the offset is the absolute minimum a worn-out
  pen can reach, this gives us the same behaviour as a new pen. The
  calculation here uses a simple approach so the actual range is
  slightly larger than 5% but it'll do.

  Previously, the lower threshold for an offset pen was the axis minimum
  but that can never be reached. So there was probably an undiscovered
  bug in there.

And fix a bunch of comments that were either wrong, confusing or
incomplete, e.g. the pressure thresholds were already in device
coordinates.

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