Color picker picks the wrong value if dialog has dark background

[strattonbrazil]

Describe the bug:
When using the chroma tool I noticed it was working very poorly. After playing with the colors I noticed the ones grabbed by the picker were much darker, which was the problem. I realized the color picker is actually pulling values off the screen--not the render view--so the dark overlay the dialog puts over the window (at least on my dialog rendering implementation) will alter the picked color. See screenshot below.

Steps to reproduce the behavior:

1. Loading OpenShot
2. Load color picker from effect property (ex. Chrome Key color)
3. Select color from render view
4. If dialog has darked the

Expected behavior:
A clear and concise description of what you expected to happen.

System Details:

• OpenShot Version: 2.5.1
• Operating System / Distro: Ubuntu 18.04

Screenshots:

[ferdnyc]

I don't know how I didn't see this one along with #3799, but PR #3815 will fix this as well.