First trell attempt
Reduced depth resolution also works for trell. Some cleanup needed.
Should be ready for testing on FRView now
More GUI elements for debug-control. 512 to 1024. Jpg works again. Background can be set. 'hold png' got a toggle.
Commented out annoying dbg msg
jpg-mode also honor the hold-png setting now
More figure-generation: dumping of images, white bg, bugfix in jpg-retain-png