WIP use cairo types

https://stackoverflow.com/questions/64625955/cairosolidpattern-is-not-of-type-goocanvas2cairopattern explains part of the problem.

I'm trying to use library via Perl bindings:

my $canvas = GooCanvas2::Canvas->new;
my $pattern = Cairo::SolidPattern->create_rgba(0, 0, 0, 0);

my $rect = GooCanvas2::CanvasRect->new(
    parent => $canvas->get_root_item,
    x => 0, y => 0, width => 100, height => 100,
    'fill-pattern' => $pattern,
    'line-dash'    => GooCanvas2::CanvasLineDash->newv([5, 5]),
    'line-width'   => 1,
    'stroke-color' => 'black',
);

This fails with the error Cairo::SolidPattern=SCALAR(0x558de4acb260) is not of type GooCanvas2::CairoPattern. The problem here (as far as I understand) is that goocanvas wraps cairo types with its own typedefs, which are picked up by the gobject-introspection compiler to produce an imcompatible boxed glib wrapper type for the same underlying C type. The other copy of the type comes from gobject introspection itself.

https://stackoverflow.com/questions/64625955/cairosolidpattern-is-not-of-type-goocanvas2cairopattern has more details.

Another part of the problem is that I want to call $canvas_image->get('pattern')->set_filter('nearest') but the returned GooCanvas2::CairoPattern doesn't have set_filter or any other methods. This patch fixes both these issues.

But before this can be merged, I have several questions:

1. What's the correct branch to merge this into? master also needs to be fixed (actually, same applies to !8 (merged))
   • switched to goocanvas-3.0 branch
2. Same should probably be done to the other wrapped types here. I started from Pattern because that's what I got blocked on in my project.
3. Re compatibility. Does anything rely on the GooCanvas.CairoPattern types? Instead, if necessary, I can add a new set of properties and leave the pattern one as is. E.g. they would be fill-cairo-pattern and stroke-cairo-pattern in addition to existing convenience properties fill-color and fill-color-gdk-rgba.