Don't convert signed to unsigned when we need all 32bit

The fast scale methods qt_scale_image_16bit and qt_scale_image_32bit
were incorrectly assigning an unsigned value to a signed variable, which
broke their support of images with dimensions from 32384 to 65535.
Images with dimensions higher than that are already using another code-
path.

Task-number: QTBUG-40297
Change-Id: I8c971889e069381224cea2befbb5c66cd93ea5c2
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>