First part of font refactoring. These changes simplify a few things
but don't do much on their own, they just make it easier to
implement the work to come (web fonts, performance improvments
in terms of font loading and memory usage).

- Font identifier on Linux/Android is now the font file path.
  This is a temporary measure, but simplifies things a lot for now.
- Remove FontListHandleMethods trait in favour of free functions.
- FontList::refresh() has no knowledge of FontFamily etc. Instead it takes
  a closure that the caller provides.
- FontList::load_variations_for_family no longer creates the font
  handle. Instead it takes a closure and provides the name of the font
  identifier for the variations it finds.
- Remove path_from_identifier() - it's no longer required.
- create_font_from_identifier() takes an Option<Style>, allowing it to be
  used to create fonts for family matching purposes where the font size is
  not important.

Tested on Linux + Mac. Builds on Android but not able to confirm it's working correctly.
Merge pull request #2751 from glennw/font-refactor-1

First part of font refactoring. These changes simplify a few things
Make compositor_data.rs conform to 100 column line lengths
Disable rust-phf until cross compile / make check issue is sorted.
Merge pull request #2757 from glennw/disable-phf

Disable rust-phf until cross compile / make check issue is sorted.
Merge pull request #2756 from zwarich/compositor-100-lines

Make compositor_data.rs conform to 100 column line lengths
Change a `fold` to an `any`
Rename a local closure in `get_buffer_request`
Merge pull request #2759 from zwarich/cleanup-get-buffer-request

Cleanup `get_buffer_request`
Stop calling Scene::set_transform

The Scene::set_transform method was only introduced because of an old
rustc bug around mutating properties across crates. Now that the rustc
bug is fixed, we can stop calling this method.
Fix some exported private type warnings.
Fix a deprecated phase warning.
Remove some unused fields.
Mark some unused fields.
Remove some deprecated into_owned calls.
Merge pull request #2762 from Ms2ger/warnings

Fix some build warnings. r=me
Update to latest rust-layers
Merge pull request #2755 from mrobinson/remove-common

Update to latest rust-layers
Fix 100-column violations in compositor.rs
Replace bare constructor functions with 'new' static member functions in rust-azure.