lwcolor: Fix non mod4 width LW48->RGB24 conversion.
lwcolor: Switch to integer version of LW48->RGB24 conversion using full bits.
lwinput: Split some simd related codes into files.
lwcolor: Add SSE4.1 version of LW48->RGB24 conversion.
lwcolor: Add SSE4.1 version of LW48->YUY2 conversion.
lwcolor: Remove the duplicated definition of CLIP_BYTE.
lwcolor: Add 'void' to the definition of func_init().

According to ISO/IEC 9899:1999 6.7.5.3 Function declarators (including prototypes),
An empty list in a function declarator that is part of a definition of that function specifies that the function has no parameters.
The empty list in a function declarator that is not part of a definition of that function specifies that no information about the number or types of the parameters is supplied.

And, 6.11.6 Function declarators,
The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature.

Therefore, in the range of C99, void func(){...} is equal to void func(void){...} while void func(); is not equal to void func(void);.
However, thinking about the future, this change is preferable.
lwinput: Create black background for LW48.
AviUtl: Document LW48 in further detail.
lwcolor: Make LW48->YUY2 and LW48->RGB conversion multi-threaded.
AviSynth/README: Fix function formats.
libavsmash: Always try to set bits_per_coded_sample from summary for video stream.
Merge branch 'plain' into build-msvc

Merge revision: r632
Conflicts:
	AviUtl/colorspace_simd.c
	AviUtl/lwcolor.c
	AviUtl/video_output.c