fdo#74702 New abstract function UsePolyPolygonForComplexGradient()

A new protected abstract function has been introduced only for
complex gradients. As it stands, we currently need to work out if
we should use a PolyPolygon or a Polygon because, as the comments
say:

    // Determine if we output via Polygon or PolyPolygon
    // For all rasteroperations other then Overpaint always use
    // PolyPolygon, as we will get wrong results if we output multiple
    // times on top of each other.
    // Also for printers always use PolyPolygon, as not all printers
    // can print polygons on top of each other.

Interestingly, the next line is either wrong or expressed badly,
because the check uses PolyPolygons when a VirtualDevice is in use:

    // Also virtual devices are excluded, as some drivers are too slow.

Therefore, I've removed that comment as it seems rather misleading.

Change-Id: Ic496284cb2be8e7e2d348eae76aeeec994e1029c
Reviewed-on: https://gerrit.libreoffice.org/8802
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>