Add Paint.hasGlyph method

This patch adds a method to determine whether a typeface has a glyph
to support a particular Unicode character or sequence.

The implementation is based on shaping the text and checking for the
presence of .notdef glyphs, and counting the number of glyphs in the
ligature case. In the case of variation selector control characters, it
currently just returns false because there is no variation selector
support yet (Mongolian Free Variation Selectors work because they're
shaped with GSUB instead).

Change-Id: I8ee4ae0d6e81a0ac57f961eb02dcc35aabd87042