r291 added support for building on platforms where subpixel support

r291 added support for building on platforms where subpixel support
has been compiled out of FreeType. However, it turns out that there is
a common situation in which we build on platforms with subpixel
support, but have to run without it: people downloading Chromium from
the buildbots.

Because we this, we need a runtime solution. Modifying the conversion
routines for this seems suboptimal because the extra branches will
slow down everyone. So we probe subpixel support at runtime by trying
to install a low-pass filter. If we fail at this, then we map all the
SkScalerContext::Rec structures to remove subpixel mode.

[evan, 2009-07-28 18:35:18]

LGTM

Might wanna make the first line of hte review better