Don’t accept Graphite features without a setting

Now that I understand Graphite a bit better, I can see why the original
code ignored features like:

  \font\krishnaalt="Krishna Tamil/GR:Classical Orthography"

where it should be specified as:

  \font\krishnaalt="Krishna Tamil/GR:Classical Orthography=True"

I thought i’d accept the first form to mean an OpenType-like feature
activation, but this makes no sense in Graphite, quoting Matrin on the
mailing list:

 “There is no OpenType concept of activation or not of a feature. Think
  more along the lines of AAT or user features. A feature is just a
  value setting within the font that the graphite code can query to
  change how it behaves. Thus all features are activated, it's just what
  their value is.”

So the user really has to provide a feature setting (value) to alter the
existing one.
Fix \XeTeXisdefaultselector with Graphite fonts