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.
Merge pull request #2775 from glennw/font-cache

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