Test picture generation reference

Contents:

• vc2_conformance.picture_generators: Picture generators
• vc2_conformance.dimensions_and_depths: Picture dimension and depth calculation
• vc2_conformance.color_conversion: Color conversion routines
  • High-level API
  • Low-level API
  • Additional utility functions
  • Color parameter sanity checking
• vc2_conformance.file_format: Picture file format I/O