The Image Trace panel had no way to compare a trace with its source
image except changing View and changing it back.
The panel now has an eye button to the right of View, enabled with an
Image Trace object selected. While it is held down, the canvas draws the
selected Image Trace objects with their source image, whatever their
View, and draws their View again on release. The document, its View and
the undo history are unchanged. The renderer takes the objects in
RenderOptions::trace_source, which is also part of the canvas frame key
and the raster-effect cache key.