Respect FC_MATRIX and FC_EMBOLDEN as extra font parameters.

Respect FC_MATRIX and FC_EMBOLDEN as extra font parameters.

A font consists of a set of data and a set of parameters to that data.
For example a ttc font consists of the full font data parameterized by
the index. In addition to the index, FontConfig allows specifying a
matrix and embolden flag. In the future there may also be additional
parameters of this sort, for example which color palette to use.

This does not provide a way to serialize these parameters.
Adding this here provides a nice place to experiment with doing so.

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Review URL: https://codereview.chromium.org/1890533002