Support subpixel text when FreeType is built without such support.

Support subpixel text when FreeType is built without such support.

On Fedora, at least, FreeType is built without subpixel support. It
tries to simulate subpixel mode by replicating the gray-values three
times. However, it fails to simulate the actions of the low-pass
filter and so doesn't add border around the glyphs. This patch adds a
compile-time constant, which is the number of bytes of filter border.
If this value is zero (because FreeType is simulating subpixel
support), then we add a transparent border ourselves when transforming
from FreeType rendered glyphs to masks.

http://codereview.appspot.com/96171

git-svn-id: http://skia.googlecode.com/svn/trunk@291 2bbb7eff-a529-9590-31e7-b0007b416f81