gestures: fix disambiguation between two finger pinch and scroll

gestures: fix disambiguation between two finger pinch and scroll

The changes introduced in b5b6f835af2e99b6a4e64fb0174ab551ed141763 to
add support for hold gestures introduced a regression:

The mechanism that was in place to improve the disambiguation between
two finger pinch and scroll during the beginning of the gesture stopped
working and instead a bug warning was printed on the log.

Fix the regression by allowing to go from the scroll state to the pinch
state.

Fix #726

Signed-off-by: José Expósito <jose.exposito89@gmail.com>