Add Multiply blend mode for D2K move-flash and shroud.
Move ShroudRenderer into a trait.
New shroud renderer. Fixes #2162. Fixes #3024. Fixes #4034.

Uses the original tile sprites in C&C and D2K and uses a smoother transition in all mods.
Update CHANGELOG.
Merge pull request #4287 from pchote/shroud

Rewrite shroud renderer.