QRasterPaintEngine::alphaPenBlt slow path does not handle useGammaCorrection

QRasterPaintEngine::alphaPenBlt does not care about gamma correction when ending up in the slow code path (!s->flags.fast_text).

This is a problem on macOS, where default text colors have a non 100% alpha, so we end up in the slow path, but the glyphs we produce (and get from CoreText) expect to be linearly blended (useGammaCorrection == true).