Text's alpha channel is ignored with ClearType antialiasing when using Qt::WA_TranslucentBackground

When the translucent background is enabled, text is drawn as if it "inherits" the alpha of the background, so you only get the color applied when blending. A workaround is to use D2D1_TEXT_ANTIALIAS_MODE_GRAYSCALE instead.

We can special-case this in the paint engine, but are there any painter hints we can use to fallback to grayscale?