fonts: Use FontInstanceFlags::EMBEDDED_BITMAPS for color fonts on MacOS

[mrobinson]

This flag ensures that these fonts are rendered full color in WebRender,
allowing for full color emoji.

---

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• There are tests for these changes, but only the manual test added here.
  This is tricky to test via WPT because color fonts are sometimes bitmap
  fonts and sometimes fonts with COLR layers. In addition, we don't run tests
  for MacOS on the bots. The hope is that there can be good automated tests for this
  for the Linux implementation.

[mrobinson]

Before this change:

After this change:

There's more work to do here, but this is one step on the way to proper emoji support on MacOS and other platforms.

[mukilan]

We are first instantiating the Font with the default FontInstanceKey and then overriding it again with a valid one - is this because the a Font instance is needed to first get the flags to be passed to WebRender in order to get the key from it?

In the future and not necessarily in this same PR, would it be possible to get the flags from the template instead of the Font instance?

[mrobinson]

I think it would be hard to get this from the FontTemplate, but there's a possibility that we could create the PlatformFont first and get it from there. The other option is we could move the FontInstanceKey out of the font or make it an Option. There could conceivably be some cases where we want to keep the font cached, but also not have it in WebRender.