Fix rotation gesture start

This patch makes the rotation gesture start more smoothly instead of
abruptingly start with an initial rotation equal to starting angle
threshold, once this threshold angle has been trespassed.

Change-Id: I2be52fa53f50151b9b87244a1b53ca4105121a8b
Reviewed-by: Bruno de Oliveira Abinader <brunoabinader@gmail.com>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>