2004-12-11  Michael Natterer  <mitch@gimp.org>

	* app/core/gimpdrawable-preview.[ch]: added new function
	gimp_drawable_get_sub_preview() which returns a scaled preview of
	a part of a drawable.

	(gimp_drawable_preview_scale): made it work with srcPR.x and
	srcPR.y being != 0.

	* app/core/gimpimage-preview.c (gimp_image_get_new_preview)
	* app/widgets/gimpviewrendererdrawable.c
	(gimp_view_renderer_drawable_render): if the area of the drawable
	preview is more than 4 times larger than the drawable itself (evil
	heuristic, but seems to work fine), use above function to get a
	sub-preview of the drawable instead of getting an insanely large
	preview of the whole drawable just to use a small part of it.
	Fixes bug #142074.

	* app/core/gimpimage-preview.c (gimp_image_get_new_preview):
	optimized by skipping layers which do not intersect with the
	canvas.