A new (internal only currently) API, _clutter_actor_queue_clipped_redraw
can be used to queue a redraw along with a clip rectangle in actor
coordinates. This clip rectangle propagates up to the stage and clutter
backend which may optionally use the information to optimize stage
redraws. The GLX backend in particular may scissor the next redraw to
the clip rectangle and use GLX_MESA_copy_sub_buffer to present the stage
subregion.

The intention is that any actors that can naturally determine the bounds
of updates should queue clipped redraws to reduce the cost of updating
small regions of the screen.

Notes:
» If GLX_MESA_copy_sub_buffer isn't available then the GLX backend
  ignores any clip rectangles.

» queuing multiple clipped redraws will result in the bounding box of
  each clip rectangle being used.

» If a clipped redraw has a height > 300 pixels then it's promoted into
  a full stage redraw, so that the GPU doesn't end up blocking too long
  waiting for the vsync to reach the optimal position to avoid tearing.

  » Note: no empirical data was used to come up with this threshold so
    we may need to tune this.

» Currently only ClutterX11TexturePixmap makes use of this new API. This
  is done via a new "queue-damage-redraw" signal that is emitted when
  the pixmap is updated. The default handler queues a clipped redraw
  with the assumption that the pixmap is being painted as a rectangle
  covering the actors transformed allocation. If you subclass
  ClutterX11TexturePixmap and change how it's painted you now also
  need to override the signal handler and queue your own redraw.

  Technically this is a semantic break, but it's assumed that no one
  is currently doing this.

This still leaves a few unsolved issues with regards to optimizing sub
stage redraws that need to be addressed in further work so this can only
be considered a stepping stone a this point:

» Because we have no reliable way to determine if the painting of any
  given actor is being modified any optimizations implemented using
  _clutter_actor_queue_redraw_with_clip must be overridable by a
  subclass, and technically must be opt-in for existing classes to avoid
  a change in semantics. E.g. consider that a user connects to the paint
  signal for ClutterTexture and paints a circle instead of a rectangle.
  In this case any original logic to queue clipped redraws would be
  incorrect.

» Currently only the implementation of an actor has enough information
  with which to queue clipped redraws. E.g. It is not possible for
  generic code in clutter-actor.c to queue a clipped redraw when hiding
  an actor because actors have no way to report a "paint box". (remember
  actors can draw outside their allocation and actors with depth may
  also be projected outside of their allocation)

  » The current plan is to add a actor_class->get_paint_cuboid()
    virtual so actors can report a bounding cube for everything they
    would draw in their current state and use that to queue clipped
    redraws against the stage by projecting the paint cube into stage
    coordinates.

» Our heuristics for promoting clipped redraws into full redraws to
  avoid blocking the GPU while we wait for the vsync need improving:

  » vsync issues aren't relevant for redirected/composited applications
    so they should use different heuristics. In this case we instead
    need to trade off the cost of blitting when using glXCopySubBuffer
    vs promoting to a full redraw and flipping instead.