Images wastes time on prescaling before OpenGL painting

WebCore prescale images under cirtain circumstances to work-around poor
downscaling in the raster QPainter. This is however unnecessary and
harmful to performance when using a OpenGL paint engine.

This patch makes sure the prescaling logic is only used for the raster
paint engine.

Change-Id: I321894ee6adec097d827e9997ebf951960ee91a4
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>