Fix fillRect on RGBA8888 images

The fill color was not correctly converted before being filled into
RGBA8888 images. This patch adds a function with convertion and
adds tests for it to tst_qpainter.

Change-Id: If8b0e6db38b2794a60301842e25f377eb7216796
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>