Add bindings for Cairo.Region

[cameronwhite]

This also adds some additional wrapper code for Cairo.RectangleInt in order to implement the unit tests (e.g. constructing a rectangle, comparing equality, and field accessors) - this is the main thing I'd like to discuss since it's a more general topic on the generator side of things!

The extra level of indirection to access the fields (RectangleInt vs RectangleIntData) feels kind of awkward - my current changes have some rough code that attempts to hide this. I feel like it would be more natural if the generator could just directly emit a struct type here, since on the C side this is a plain struct rather than an opaque / handle type

I think it might also make something like cairo_region_create_rectangles() simpler to wrap as well since the memory layout would match
The old cairo bindings from Mono / GtkSharp also made this a struct type, so changing from value to reference semantics would also make it tricky to port existing code correctly

[badcel]

Should wait until #622 is fixed to rely on improved struct generation.

[cameronwhite]

I've rebased this on top of the latest changes - the RectangleInt struct is much more usable now without any manual bindings 👍
The two awkward things I encountered are:

• Constructing it requires new RectangleInt(Internal.RectangleIntManagedHandle.Create()) for now, as was noted in the release notes
• Equality comparison doesn't work, I think because it includes the Handle member. So code which could have been written as rect_copy.Should().Be(rect_init) requires comparing all the members explicitly. Mostly just an annoyance for writing unit tests :)

[badcel]

Ah good point! I will add some equality check to the generator which just compares the pointers.

edit: Or perhaps we do the member comparison? I think it could even be relatively easy. It would be nice to read your opinion in #1000.