DirectWrite doesn't count tabs as trailing whitespace in single-line layout?

Nobody has claimed this yet.

This is very much related to #411, but slightly more specific.

In #409 I added the following test:

```rust
fn line_test_tabs() {
let line_text = "a\t\t\t\t\n";
let mut factory = make_factory();
let layout = factory.new_text_layout(line_text).build().unwrap();
assert_eq!(layout.line_count(), 2);
assert_eq!(layout.line_text(0), Some(line_text));
let metrics = layout.line_metric(0).unwrap();
assert_eq!(metrics.trailing_whitespace, line_text.len() - 1);
}
```

and this is now passing in coretext; previously it was running out of layout space and dropping some of the text.

DirectWrite currently *fails* this test, reporting its trailing whitespace length as '1'.

I think this is a matter of different expectations around trailing whitespace for these two APIs, but I can't investigate this week, since I don't have access to a windows machine.

Start with the line_test_tabs test described in the issue and the factory.new_text_layout call, then trace the DirectWrite layout implementation that supplies line_metric(0). Compare its handling of tab characters with the expected trailing_whitespace value and confirm the test passes without dropping text.

Written by the indexing model from the issue text.