Fix drawing vertical gradients in RGBA8888 formats

The RGBA8888 formats was incorrectly using the qt_gradient_quint32
which is argb specific. This caused vertical gradients but only
vertical gradients to be drawn incorrectly.

This changes the RGBA8888 formats formats to use the generic gradient
method and renames qt_gradient_quint32 to qt_gradient_argb32 to
indicate its limitation.

Change-Id: Ia1cd48ca7f4f78b64f31d6263e81cd8ac3b0954e
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>