Fix problem with glyph cache and textScaleX property

Glyphs drawn with paints that had different textScaleX values were not
being recognized as different, so the glyph cache was being used regardless
of different scaleX values. This change adds the scaleX attribute to the native
Font object to allow the cache to distinguish between this difference and cache
accordingly.

Change-Id: I5d8fc26d47460b27dc8e373a473d46b2f1b8dc30