Auto merge of #15096 - vvuk:custom-font-files, r=emilio

Implement downloadable fonts on Windows

This PR implements downloadable font support for Servo.  It depends on new changes in webrender and dwrote, and adds a dependency on the `truetype` crate for pulling out basic font information.  The original DirectWrite API does not provide an easy way to query font information direct from a `FontFace` (which is what you create from a `FontFile`).  There are new DirectWrite APIs starting with Windows 10 that allow for this, but they are Win 10+ only, and `winapi-rs` does not yet have bindings for those versions of the interfaces (specificially, `IDirectWriteFontFace3`).  The way to do this with DW is to go through a lot of pain in creating a custom collection and enumerator, add your font to your custom collection, then query the collection for its properties.

Instead, we just parse the truetype tables directly to pull out the few bits of information that we need.  The `truetype` crate is ok, but I discovered some bugs (an update needs to get pushed to crates.io before this will build).  It might be more worthwhile to just implement the tiny bit of truetype parsing that we need ourselves in Servo.

I'm guessing there are existing tests for downloadable fonts...

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