- put draw_idle in the base canvas resize event
 - re-order our resize handling and Qt's resize handling (let Qt go
   first)
 - do not call processEvents from inside of the paint event (this
   maybe the critical fix)

The resizeEvent call does put another paint onto the event queue,
however it seems that processEvents only processes events on the queue
when it is called (not events added while it is processing).  This is
not clearly documented, but makes sense as this method is for keeping
GUIs responsive during otherwise long-running tasks so it should not
be able to extend it's run indefinitely by populating the queue.

This is required because a previous commit removed the explicit
processEvents call in paintEvent as it would occasionally cause
recursive painting which may kill Qt.

This prevents flickering during window resize.  The issue is that the
resize event triggers a paint and we have marked the figure as stale
and requiring a re-draw.  Sometimes the paint from the resize will
process before the single-shot to trigger __draw_idle_agg and there
will be a size-mismatch between the QImage and the buffer we are
trying to fill it with which results in a visible flicker.