MainFrm: fix small back jump with missing index

Refs #3773
Include wmcodecdsp.h and remove duplicate GUID definitions.

Based on a similar commit of LAV Filters.
Simplify build_ffmpeg.sh now that we use out-of-tree compilation.
Use a bitcheck instead of casting.

The result of a cast from unsigned to signed isn't defined by the specs.

Fixes CID #965944.