Ensure QImage::pixel() on RGB32 images returns valid QRgb values

Currently QImage::pixel() returns the raw underlying pixel values for
RGB32. For all other pixel formats the returned value is either valid
ARGB32 or ARGB32PM.

This patch masks the undefined alpha field in RGB32 so that the return
value is well defined QRgb. It also fixes one test that relied on the
previous behavior.

Change-Id: If37463528268b7419733499d1f7bfd0d1097d21e
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>