Unresolved import `direct2d::math`

Nobody has claimed this yet.

I am (probably naively) trying to run "cargo build" on a fresh pull of `piet` and I'm seeing several errors in the build, e.g.:

```
error[E0432]: unresolved import `direct2d::math`
--> piet-direct2d/src/conv.rs:3:15
|
3 | use direct2d::math::{Matrix3x2F, Point2F, RectF};
| ^^^^ could not find `math` in `direct2d`

error[E0433]: failed to resolve: could not find `shared` in `winapi`
--> piet-direct2d/src/lib.rs:11:13
|
11 | use winapi::shared::basetsd::UINT32;
| ^^^^^^ could not find `shared` in `winapi`

error[E0433]: failed to resolve: could not find `um` in `winapi`
--> piet-direct2d/src/lib.rs:12:13
|
12 | use winapi::um::dcommon::D2D_SIZE_U;
| ^^ could not find `um` in `winapi`
```

I assume that as a new user of rust I just don't know what I'm doing :)

Start by reproducing the reported errors with cargo build on a fresh piet checkout. Inspect the imports in piet-direct2d/src/conv.rs and piet-direct2d/src/lib.rs, then determine why direct2d::math and the winapi modules are unavailable. Done means the project builds successfully with these imports resolved.

Written by the indexing model from the issue text.