Zooming transforms results in blurry backgrounds

An optimization to speedup pattern transforms results in blurry
background images when using CSS transforms.

This patch disables the optimization when QPainter already has a
transform, and the optimization would be both wrong and probably slower.

Test: svg/as-background-image/svg-transformed-background.html

Task-number: QTBUG-38588
Change-Id: I58739e21fd925048bc91f0ac973fae2036bcf713
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>