Fix fast painting of clipped text on non RGB32 formats

QRasterPaintEngine::alphaPenBlt makes the assumption that a device with
bit depth 32 has optimized alphamapBlit and alphaRGBBlit routines.
This will fail on RGBA8888 format resulting in some text not being
rendered.

Change-Id: Ia7d88bb0e3094894affceda1acc42396b67b3677
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>