Image vectorscope is getting some visual updates (#116974), here make the
Sequencer vectorscope match the look fairly closely:
- Use same scaling factor for both U & V, instead of trying to use all
the texture space.
- Instead of drawing hexagonal area between color primary "pure" and
"75% saturation safe" points, draw a circle with colored background
and five inner rings.
- Indicate "75% saturation primaries" as wire rectangles with a text label.
Images in PR.
Pull Request: https://projects.blender.org/blender/blender/pulls/117738
5ff0feee52