Mac: pixmaps dont treat alpha exactly the same as other platforms

Mac: pixmaps dont treat alpha exactly the same as other platforms

From fixing qwidget autotest, it seems to be the case that filling
a pixmap with a solid color on mac changes the hasAlpha status. This
is different from X11 (and raster engine). That is, the following
code will produce different result:

QPixmap pixmap(100, 100);
pixmap.setAlphaChannel(QPixmap(100, 100));
qDebug() << "Has alpha after setting channel?" << pixmap.hasAlpha();
pixmap.fill(Qt::red);
qDebug() << "Has alpha after fill?" << pixmap.hasAlpha();

So, this patch remove the possibility of converting an already alpha
supporting pixmap to non-alpha upon fill. NB: somewhat related, the
following code works the same on both x11, raster and mac (prints
'false'), but still seems wrong:

QPixmap pixmap2(pixmap);
qDebug() << "pixmap2 has alpha initially?" << pixmap2.hasAlpha();

Rev-By: Fabien Freling