ok, i modified *way* more files than CVS is telling me....

- add canvas_init machinery (tile_manager_validate equivalent)
- fix bugs in compositing code
- layers/channels dialog, previews
- all color handling should be fairly precision independent now
- replaced gimage_type and gimage_base_type with Tags
- remove concept of flat vs. layered gimages

ray lehtiniemi <rayl@netrover.com>