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.
Updated Simplified Chinese translation
MainFrm: fast exit play command while opening
MainFrm: don't paint m_pCAP until the rect is set
MainFrm: don't pause the graph in graph thread

Interface repainting may be sluggish after it, instead, enter play or
paused state from main thread after everything is repainted
MainFrm: set icon before constructing the graph
MainFrm: add new MLS_FAILING load state
MainFrm: use strongly-typed enum for load states