Another ugly hack to make bidi cursor work with Core Text

Another ugly hack to make bidi cursor work with Core Text

If the text is wrapped with LRE/LRO/RLE/RLO override/embed marks,
Core Text in Mac OS X 10.5 doesn't produce an empty glyph at the
beginning of the glyphs (while it does in Mac OS X 10.6), thus
we need to prepend an empty glyph here, otherwise cursor position
calculation will consider the first two characters as a ligature
of the same glyph.

Reviewed-by: Eskil
(cherry picked from commit a36ac6c34bafa801c2c30d76f59e4a3594efc4d5)