Use correct bounding box for MultibandFootprint

The original version of this code used the bounding box of the
spanset, not the mask that was used to create the spanset.
When the mask contains empty rows the two will differ, which
caused some sources to be placed incorrectly in the footprint.

python/lsst/meas/extensions/scarlet/source.py

@@ -142,6 +142,6 @@ def modelToHeavy(source, mExposure, blend, xy0=Point2I(), dtype=np.float32):
     # Create the MultibandHeavyFootprint
     foot = Footprint(spans)
     foot.setPeakCatalog(peakCat)
-    model = MultibandImage(mExposure.filters, model, spans.getBBox())
+    model = MultibandImage(mExposure.filters, model, valid.getBBox())
     mHeavy = MultibandFootprint.fromImages(mExposure.filters, model, footprint=foot)
     return mHeavy