improve bitmap font support (FreeType only)

This commit improves SkFontHost_FreeType's support for bitmap fonts,
adding a number of features:
- Intelligent bitmap strike selection.
- Inter-strike bitmap font scaling.
- Colour bitmap font support (FreeType 2.5.0+).

BUG=
R=reed@google.com

Review URL: https://codereview.chromium.org/23684041

git-svn-id: http://skia.googlecode.com/svn/trunk@12607 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed