This is all work toward #677, attempting to avoid running the pipeline when...
This is all work toward #677, attempting to avoid running the pipeline when doing edit operations. Because the pipeline must be run for generating thumbnails, that run is not avoided with this check-in. (This may be addressed with background threads.) This check-in avoids the pipeline when possible with current architecture, and attempts to reuse an old pixbuf if the user cancels the edit tool.
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