Re-introduce paint in Windows backend

[bzm3r]

Commented out by unpiet process.

See:

glazier/src/backend/windows/mod.rs

Line 20 in d306005

// pub mod dcomp;

and:

glazier/src/backend/windows/mod.rs

Line 25 in d306005

//pub mod paint;

[raphlinus]

I suspect that dcomp should probably not be done here, as I don't believe raw_window_handle can eventually plumb down to CreateSwapchainForComposition; I believe it only the CreateSwapChainForHwnd method can be wired up. That said, I think plumbing WM_PAINT to paint() probably does seem like a good idea, as a way to trigger repaint cycles that end up in a Present call.

Composition is way out of scope for now, but might be on the long term roadmap. See raphlinus/raphlinus.github.io#77 for more thoughts on that.