[cairo] glitz_drawable_swap_buffer_region slow?

[stefan kersten]

i'm having a problem with the glitz backend and
double-buffered drawables: to avoid having to redraw the
whole window i use a damage rect, but when the damage is
smaller than the window size, performance decreases quite a
bit.

iow this
 
  glitz_drawable_t* drawable = glitz_surface_get_drawable(m_glitz);
  glitz_box_t damage = { x, y, x+w, y+h };
  glitz_drawable_swap_buffer_region(drawable, 0, 0, &damage, 1);

is a lot slower than this

  glitz_drawable_t* drawable = glitz_surface_get_drawable(m_glitz);
  glitz_drawable_swap_buffers(drawable);

any ideas as to what might be happening?

<sk>