Move libavsmash.c/h into common directory.

libavsmash.c is expected that it is usable for both C and C++.
lsmashinput: Cosmetics about Libav-SMASH video handler.
lsmashinput: Cosmetics about Libav-SMASH audio handler.
lsmashsource: Cosmetics about audio handler.
Integrate resampler functions and moved into common directory.
Integrate Libav-SMASH audio decode/output handler and moved into common directory.
Integrate Libav-SMASH video decode handler and moved into common directory.
vslsmas: Simplify background black generation.
vslsmas: Avoid calling scaler if not needed.
vslsmas: Cosmetics about video scaler handler.
vslsmas: More cosmetics.
vslsmas: Component reorder.

Now, ARGB/RGBA/ABGR/BGRA -> RGB24 .
Merge branch 'plain' into build-msvc

Merge revision: r407
Conflicts:
	AviUtl/audio.h
	AviUtl/libavsmash.c
	AviUtl/libavsmash_input.c
	AviUtl/resample.c
	VapourSynth/libavsmash.c
	VapourSynth/lsmashsource.c
	VapourSynth/video_output.c