Issue #7960: Fadeout option in "Filters > Animation > Burn-In" doesn't work

Attempts to resolve issue with Burn-In Animation filter reported in #7960 (closed). Changes discussed in this comment: #7960 (comment 1406068)

In addition, "Prepare for GIF" option appeared to be broken as well. It used an undeclared/undefined variable "img-out" to store the result of the plug-in-animationoptimize operation. Additionally, it tried to use the temporary bl-layer rather than the finalized blended-layer variable. Both issues are resolved in this patch.

This issue exists in both 2.10 and 2.99. If approved, these changes should be easy to port to the other branch.