Fix libswscale compilation with --disable-optimizations on x86-32.

Fixes ticket #2477.
cmdutils: make the "-help filter=" output less confusing

Example:
Filter overlay
  Overlay a video source on top of the input.
    Inputs:
       #0: main (video)
       #1: overlay (video)
    Outputs:
       #0: default (video)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>