Marshalled PDB procedures into script-fu can now convert a filter ID
into the proper object. For instance, here would be the code to append a
new gaussian blur filter to the drawable with ID 2 (with specific
settings), then making invisible:
```script-fu
(define filter (gimp-drawable-append-new-filter 2 "gegl:gaussian-blur" "hello" LAYER-MODE-COLOR-ERASE 1.0 "std-dev-x" 20 "abyss-policy" "none"))
(gimp-drawable-filter-set-visible filter FALSE)
(gimp-drawable-update 2 0 0 -1 -1)
```
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