touchpad: avoid motion events when moving one finger into AREA

If a 2fg scroll motion starts with both fingers in the bottom button area and
one finger moves into the main area before the other, we used to send motion
events for that finger. Once the second finger moved into the main area the
scroll was detected correctly but by then the cursor may have moved out of the
intended focus area.

We have two transitions where we may start sending motion events: when we move
out of the bottom area and when the finger moves by more than 5mm within the
button area. In both cases, check for any touches that are in the
bottom area and started at the 'same' time as our moving touch. Mark those as
'moved' to release them for gestures so we get the right finger count and
axis/gesture events instead of just motion events.

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