Clarify color space handling (option 2; hardware decoders do what they want) - #957

- Define internal color space on media resources, exposing unmapped or
  unspecified components as null on VideoColorSpace.
- Select the color space with the most specified primaries, transfer,
  and matrix components between VideoDecoderConfig and the codec during
  decoding, breaking ties in favor of VideoDecoderConfig.
- Allow codec implementations to modify or fill in color space
  components during decoding and encoding.
- Remove heuristic guessing from Pick Color Space and fall back to sRGB
  or REC709 at render time when primaries, transfer, or matrix are
  unspecified.

Fixes #940.