vslsmas: Add 'format' option.

This option forces output pixel format defined in VapourSynth.
This option is available if 'variable' is false.
Currently, the following formats are supported.
	YUV420P8
	YUV422P8
	YUV444P8
	YUV420P9
	YUV422P9
	YUV444P9
	YUV420P10
	YUV422P10
	YUV444P10
	YUV420P16
	YUV422P16
	YUV444P16