app: in GimpHistogramEditor, don't block when creating bg histogram
In GimpHistogramEditor, when the drawable preview is frozen, don't duplicate the main histogram as the bg histogram if calculation is still ongoing, since this will block until histogram calculation is complete. In particular, this creates a noticeable stall when beginning a paint stroke while the histogram is being calculated. Instead, defer the creation of the bg histogram to the completion of the calculation of the main histogram.
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