With this patch the histogram and vectorscope data for the image is
only computed when the widgets are visible to the user and therefore
saves some processing time when this information is not required
to be computed.
Vectorscopes are a standard tool in professional video/film color
grading. *Very* simply, the Vectorscope shows image colors with hue as
the angle and saturation as the radius; brightness for each point in the
scope is determined by the number of "color vectors" at that point. More
specifically, the Vectorscope shows a 2D UV histogram of the image,
where U and V are the chroma ("color") channels of the image.
Co-authored-by: MacDue <macdue@dueutil.tech>
