Generalize tts:fontVariantPosition

[plehegar]

In order to support Japanese (and other CJK) usage, it is useful to support selection of other axes of glyph variation besides position (super/sub). In particular, selection of full width and half width variants, as supported by the OpenType 'fwid' and 'hwid' features, as well as ruby variants, is useful. To this end, the tts:fontVariantPosition property should be renamed to tts:fontVariant, and its values enhanced to specify, at a minimum:

• super vs sub (script) forms
• full vs half width forms
• ruby forms

Specifically, propose the value of the generalized property to adhere to the following:

tts:fontVariant
: normal
| [ super | sub ] || [ full | half ] || ruby

When super or sub is specified, then the 'sups' or 'subs' OpenType feature is enabled. When full or half is specified, then the 'fwid' or 'hwid' OpenType feature is enabled. When ruby is specified, then the 'ruby' OpenType feature is enabled.

(raised by Glenn Adams on 2015-02-23)
From tracker issue http://www.w3.org/AudioVideo/TT/tracker/issues/374