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.
The histogram is perfectly fine with being drawn at any size, but the
code currently fixes its height to 65. Once the histogram is in a
subclass and several GML things around it change, the fixed height
breaks, so we move the height specification to GML. Additionally, the
container is specified to shrink as much as possible, alleviating a
hard-coded UI size. The user can now change histogram height in GML,
which is a lot more obvious.
This adds a simple histogram widget that visualizes the rgb-channels
and brightness for a given image. When hovering over the image it will
indicate what brightness level the pixel at the mouse position has.
