Composition support

[Ciantic]

This is just preliminary PR for #29, not to be merged yet.

I have to probably refactor this a bit, but it's now initial proof of concept.

It's very possible that this d3d12 repo doesn't need to have examples, but I had to do one to test my code before trying to hack the bigger repo gfx.

---

Includes a simple examples/composition.rs that renders D3D window with test alpha channeled transparent background color to see it works. Run example with:

cargo run --example composition --features implicit-link

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[kvark]

this isn't required to demonstrate the composition. Just clearing the screen (or doing multiple clears of sub-rectangles) would do the trick, and would end up with less code here.

[Ciantic]

Thanks for looking into this!

I have no idea what code I can remove from this yet. Or how to "clear sub-rectangles"? Code in this example I basically copy & pasted blindly from gfx and microsoft's DirectX examples until I got something visible on screen. And I still have no idea what I can just "take away" and have working example.

[kvark]

this seems reasonable!
Is the "_hwnd" suffix intentional here?

[Ciantic]

It was intentional, because it also calls IDCompositionTarget::CreateTargetForHwnd with given hwnd. However the way this d3d12-rs crate is structured, I probably shouldn't be calling multiple things in one function?

[Ciantic]

Yeah, I think I will refactor this so that it's just create_swapchain_for_composition, and it doesn't call other functions. Then I will do comp.rs where I move this other stuff such as creating wrappers for these:

        // Create IDCompositionTarget for the window
        let mut comp_target = CompTarget::null();
        unsafe { comp_device.CreateTargetForHwnd(hwnd as _, 1, comp_target.mut_void() as _) };

        // Create IDCompositionVisual
        let mut comp_visual = CompVisual::null();
        unsafe { comp_device.CreateVisual(comp_visual.mut_void() as _) };

        // Set swap_chain and the root visual and commit
        unsafe {
            comp_visual.SetContent(swap_chain.as_mut_ptr() as _);
            comp_target.SetRoot(comp_visual.as_mut_ptr() as _);
            comp_device.Commit();
        }

I move each of these in own function in comp.rs and I probably need to do libloading feature for the DCompositionCreateDevice function so that it need not be linked.

Not sure , but I think that code I wrote leaks resources: IDCompositionTarget and IDCompositionVisual because WeakPtr doesn't call Release on drop.

I'm used to this kind of implementation for ComPtrs, if we don't call Release on drop, it becomes a bit cumbersome to track the life time IMO.