c_plugin: Fix regression in rffmode error handling.

The regression caused by r483 would cause it to error out even if we
didn't specify any change in the output color space.

Applies change from master @ ef81fe1
c_plugin: Remove CPUFlags stuff from the avisynth plugin

Applies changes from master @ f8207e3
c_plugin: Add FFMS_SetInputFormatV

This overrides the pixel format, color range and color space used for
the input when converting formats with swscale, for when the values
reported by lavc are incorrect, or for compatibility with other programs
that get the conversion wrong.

Applies change from master @ b058f2d