2000-05-12  Sven Neumann  <sven@gimp.org>

* gimpdrawable.c: enabled the (commented out) signal
  "invalidate_preview".

* app/layers_dialog.c: connect to the "invalidate_preview"
  signal to catch changes that need to be shown in the layer
  previews. Synthetize an expose event when a layer changes.

  Expose events are optimzed away by GTK+ if the widget is not
  visible. Therefore, previews not visible in the layers_dialog
  are not redrawn when they invalidate. Later the preview gets
  validated by the image_preview in lc_dialog but is never
  propagated to the layer_pixmap. We work around this by using an
  additional flag "layer_pixmap_valid" so that the pixmap gets
  updated once the preview scrolls into sight.

  Fixes bugs #10549, #10300 and #8787.

* app/channel.[ch]
* app/layer.[ch]: code review and indentation


--Sven