Speed up printing when drawing opaque primitives.

We only need to merge the rectangles into a region if we are painting
a non-opaque a primitive with alpha.

Performance measurements QT_print_speed_bug.zip:
Excluding patch: 244686 ms
Including patch:   5070 ms

This is an improvement of 48x (for debug build)

Task-number: QTBUG-48334
Change-Id: I03684c6e7d8a5fb039ea6477ed1a860f09e1b08c
Reviewed-by: Andy Shaw <andy.shaw@theqtcompany.com>