Some details:

* This was mostly taken verbatim from revision:5717 (before the vout
  Mozilla support was removed), though it's been edited pretty thoroughly,
  and is now much more commented.

* The "normal" vout display should be completely unaffected, since the
  plugin-relevant code paths are only taken when p_vout->p_sys->b_embedded is
  set to VLC_TRUE.  (I've tested the normal VLC.app, and it seems fine.)

* There are still some problems with the plugin when the Mozilla window is
  resized a lot.  I suspect this is due to threading issues with
  QuickDraw, but I don't know enough about QuickDraw to be sure.  Help
  with this would be very welcome.

* The original patch in revision:5717 optimised the plugin display
  slightly, by using a mask (clipping region) so that QuickDraw only
  updated the plugin's area of the dirty region.  I elected not to use
  a mask, since I thought the extra code complexity (i.e. lots more if()
  branches) isn't worth the incremental speedup.  (If, in fact, there was
  a speedup at all -- the extra overhead induced by calculating the
  intersection of the dirty region with the mask may have offset any
  benefits: only benchmarks will tell ...)