in drawPixmap(source, sourceRect) the source rect is in native pixels.
In this code the size was scaled by the DPR, but the location was not.
blendRect is the contents of a selection rectangle, offset from the
bounding darker blue rectangle. Hence we haven't really noticed.
This completely fixes the rectangle when the scale == 2.0/3.0
Some smaller artifacts remain on fractional scale factors, which is a
separate code bug with the same symptoms.
CCBUG: 386111