Bidirectional display in GUI sessions

I have committed to the Emacs development trunk revno 99950, which
adds implementation of bidirectional display in GUI sessions (as well
as a few fixes for display of R2L lines on TTYs).

I could only test this on MS-Windows; I have no easy access for
development on an X-based machine.  Fortunately, with the exception of
one function, all the code is in terminal-independent parts of Emacs
display engine, so it should work (or break ;-) the same on any type
of display.

I hope I didn't break anything in non-bidi sessions.  All the changes
are conditioned on variables that are not set unless you set
bidi-display-reordering non-nil.  But I was wrong about this in the
past...

Last, but certainly not least: Thanks to Gerd Möllmann for his kind
guidance and advice which was instrumental in making this happen.
Without his help, I would never have found such a simple and elegant
solution for implementing this feature, being quite ignorant about
internals of the Emacs GUI display.