scripts: Remove last use of SF-VALUE

This replaces SF-VALUE in burn-in-anim with SF-ADJUSTMENT, allowing us to remove SF-VALUE from the API.

I told @bootchk that this would be easy, so I felt obligated to do it. 😄

The only concern is the new value ranges for the four parameters. I set Corona and Fadeout width to the maximum size of gegl:grow, since they're used in that function. The other two, I used limits in the similar blend-in script. @ajonsson, since you're familiar with the scripts, do you have any thoughts?