Implement NativeFontHandle writing on Mac

[jrmuizel]

We can implement this using the technique here: https://searchfox.org/mozilla-central/source/gfx/2d/ScaledFontMac.cpp#255

I have patches in progress.

[jrmuizel]

servo/core-foundation-rs#127 and servo/core-graphics-rs#104 add the platform support that we'll need for this.