A trace handed in for imageTrace.make or imageTrace.makeAndExpand was
matched by its object, image, pixels and settings alone, so a trace made
in one document could stand in for the command in another open document
with the same image and settings. The trace's inputs now carry the
document's uid: TraceRun::current is false in any other document, and the
commands there drop the trace handed in and trace their own image, as one
undo step.
Test: a_trace_run_applies_only_in_its_own_document.