Import as 601

Turns out our YUV conversion is using the Rec 601 coefficients, so we
should import as same. But by definition, WebM is for the web, i.e.
computer screens. That color space (sRGB) is neither Rec 601 nor Rec
709, but it's closer to 709. We may make changes to our color science
in the future depending on that the WebM folks say.