Next stage of refactoring font system. This commit introduces
the font cache task, and adapts client code to use it. It also
cleans up some existing code paths.

- Fonts are only read once from disk while in use (they
  are discarded if the reference count reaches zero, however).
  This saves memory and prevents unnecessary reading from disk.
- It will be easier to add web font support, as all fonts
  are created and managed in a single place and the entire
  pipeline ensures that only one in-memory copy of font data
  is required.

An overview of how the pieces fit together:

FontTemplate - A structure containing everything that
is required to create (and select) font handles. This
structure is shared among all matching font handles
(via Arc).

FontTemplateData - A platform specific structure that
contains the actual font data inside a template (this is
a byte array on Linux/Android, CTFont on Mac).

FontHandle - An opaque, platform specific handle to
a font instance. Each FontHandle contains an Arc<>
reference to the FontTemplate it was created from.

FontCache - This is a separate task, that is responsible
for loading and caching FontTemplate structures. There
is one FontCache per constellation. It is only ever accessed
via the FontContext described below.

FontContext - This is the public interface to the FontCache
and is used by the layout and render code to create font
handles. These must *not* be shared between threads. There
is typically one FontContext per thread/task.
Address review comments.
Make compositor layer creation atomic

Instead of creating, sizing, and setting up layers using several
messages use only one. If the layer already exists, it will be updated
to reflect the new properties. The clip rect is still set in a separate
message, but that will be eliminated in a later commit.
Pass creation/update data for compositor layers in a struct

Use a new LayerProperties struct to hold all the data necessary to
create or update compositor layers. This reduces a lot of duplication
when passing it along.
Merge pull request #2788 from mrobinson/compositor-cleanup

Make compositor layer creation atomic
Merge pull request #2775 from glennw/font-cache

Font refactoring work - add font cache task, cleanup various code paths.