Description of the new feature/enhancement

This feature would add a setting to change the opacity of the text background when it is set. This is not the screen background which already has this feature. It is also not per-color transparency. The rationale for this is that I have an image background, but I also use vim a lot which tends to want to set a solid background.

Basically, I can do this:

Or this:

And I'd like to be able to do (roughly) this:

It'd look a little different because some solid background is still there, but that should give you the general idea.

Proposed technical implementation details (optional)

Add a profile setting backgroundOpacity. If this is lower than 1, when a cell is printed with a custom background color, that color is blended with the default terminal background instead of being fully opaque.

Bonus: backgroundBlend - Different color blend modes.

I'll also accept pointers on how to add these settings and pipe that info through to the renderer :).