Previously we only applied pressure offset handling for tablets that
supported ABS_DISTANCE. Detecting a pressure offset when the tool
doesn't actually touch the surface is easy after all.

But tablets without distance handling may also have a pressure offset,
so let's try to detect this. This is obviously harder since the pen will
always touch the tablet's surface whenever it is in proximity and thus
will always have *some* pressure applied to it.

The process here is to merely observe the minimum pressure value during
the first two strokes of the pen. On the third prox in, that minimum
pressure value is taken as the offset. If the pressure drops below the
offset, the offset is adjusted downwards [1] so over time we'll
get closer to the pen's real offset.

[1] this is already done for distance tablets too

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