Utils: making StyleHelper::drawCornerImage() dpi aware

An image passed to drawCornerImage() can have any devicePixelRatio or
rather "its own imagePixel to userPixel ratio".

drawCornerImage() calls QPainter::drawImage and now passes it a source
rect in "image pixels" and a target rect in "user pixels".

Change-Id: Ia0d6459c1570ec5d4f87568e653526f738594f6e
Reviewed-by: Marco Bubke <marco.bubke@digia.com>