Add y4mscaler, a general-purpose video scaler which operates on YUV4MPEG2

streams, as produced and consumed by the MJPEGtools such as lav2yuv and
mpeg2enc(1).

y4mscaler is meant to be used in a pipeline. Thus, input is from stdin, and
output is to stdout. The essential function of y4mscaler is to scale a
specified "active" region of the input stream (the source) into a specified
active region of the output stream (the target). Pixels outside of the active
region of the source are ignored; pixels outside of the active region of
the target are filled with a background color. The source may additionally
have a matte applied to it; pixels outside the source matte are set to a
separately specified background color.

y4mscaler correctly handles chroma sampling, and thus it can also perform
chroma subsampling conversions. The YUV4MPEG2 stream format currently only
supports 4:2:0 subsampling, although y4mscaler can process 4:1:1, 4:2:2, and
4:4:4 modes by abusing the YUV4MPEG2 format a bit.

Includes preset parameters for popular output streams: DVD, SVCD, VCD,  DV,
CVD, and SVCD/VCD still images.

PR:		ports/63962
Submitted by:	michael johnson <ahze@ahze.net>

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