Support subpixel text when FreeType is built without support.

Support subpixel text when FreeType is built without 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.

[evan, 2009-07-28 00:00:17]

as a transformation of the source to introduce a constant, it seems correct, but
the reasoning behind this is completely opaque to me.  perhaps you could improve
the review description.

http://codereview.appspot.com/96171/diff/1/2
File src/ports/SkFontHost_FreeType_Subpixel.cpp (right):

http://codereview.appspot.com/96171/diff/1/2#newcode40
Line 40: static const int kFilterBorderWidth = 3;
might be nice to copy the review description into here as a comment.