[cairo] Locking font / pen space

[Behdad Esfahbod]

It occured to me last night that instead of changing the behavior of font /
pen space locking at the they are set, we can add two new calls:

  cairo_lock_font_space()
  cairo_lock_pen_space()

These two will record the ctm at the time of the call to be used for text /
stroke until the font-space / pen-space is set again (perhaps implicitly by
cairo_restore()).

May want to add the unlock() versions too.  I don't like the fact that the
unlock is not mandatory, but I think I can live with this API and have more
confidence in it than I have in changing the semantics of existing API.

behdad