`GDrawingSurface::drawPolygon` and `GDrawingSurface::fillPolygon` for non-`std::initializer_list` arguments

[htiek]

The existing functions GDrawingSurface::drawPolygon and GDrawingSurface::fillPolygon currently only accept std::initializer_lists as arguments. This means that it's not possible to dynamically generate a list of points that make up the polygon (say, as a Vector<GPoint>) and pass that as an argument to these functions.