Distance field glyph cache refactoring.

The distance field glyph cache is now an abstract class part of the
adaptation layer. It can be implemented to define the way the glyphs are
stored in graphics memory on a specific platform. The typical use case
is for having a cross-process glyph cache. To implement a custom glyph
cache, one has to override three pure virtual functions: requestGlyphs(),
storeGlyphs() and releaseGlyphs().

The default implementation works the same way as before the refactoring
(no cross-process cache).

Change-Id: I6e231a119abbffbe36f5f0d690f0b8be0664ff4f
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>