TCY as U+FFFC

For text-combine-upright:
# For other text layout purposes, e.g. emphasis marks, text-decoration,
# spacing, etc. the resulting composition is treated as a single glyph
# representing the Object Replacement Character U+FFFC.

This should probably say that it behaves the same as an ideographic
character, not U+FFFC, since the Web-compatible line-breaking behavior
of an image forces a break opportunity regardless of surrounding text,
and we probably don't want that here.

~fantasai