#708: Do not use thumb for pinch when is used to press the clickpad

The changes made in ca3df8a0 to improve pinch detection introduced a regression:

When the thumb is used to press the clickpad it is automatically tagged as thumb and the gesture state machine does not initialize it, leaving its initial X and Y position set to 0. When another finger is put on the clickpad, the distance moved by the thumb is checked and because its initial position is 0 movement is detected.

Add an additional check to take into account only thumbs that are used in the gesture.

Fix #708 (closed)