macOS: Take application appearance into account when drawing glyphs

macOS 10.14 uses a new font smoothing algorithm that takes the fill color into
account. This means our default approach of drawing white on black to produce
the alpha map will result in non-native looking text when then drawn as black
on white during the final blit. As a workaround we use the application's current
appearance to decide whether to draw with white or black fill, and then invert
the glyph image in the latter case, producing an alpha map. This covers the
most common use-cases, but longer term we should propagate the fill color all
the way from the paint engine, and include it in the key for the glyph cache.

At the moment we do not react to changes in the application appearance,
as that seems to be buggy in general in Qt (palette/style, e.g.), and those
bugs need to be weeded before we can react to the theme change with confidence.

Task-number: QTBUG-68824
Change-Id: Ibbfd49fcf3a091e454009c08159f46b3499e2bd0
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

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