Actually three separate issues addressed. Fixes#2197.
- brush.importOldBrush() was not being called anymore, due to it being
registered as a load handler after the image was already loaded (this
was a very recent regression, less than 24 hours old).
- Each time the brush changes, the plot is redrawn twice. This was
because importing the old brush introduced floating point errors that
led to a slightly different new brush being created.
- Sometimes the image's load event wasn't firing at all. This is due to
behavior in WebKit where assigning an image's src to its existing
value is a no-op.
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