This commit enables signalling a colour matrix index using
cicp_matrix_coefficients_idx (ISO/IEC 23001-8 codec independent code
points).

It adds the following options that replace the existing colorTransform
option:

 - colourMatrix: The coded representation according to ISO/IEC 23001-8.

 - convertPlyColourspace: Enables conversion to/from RGB using the
   indicated matrix.

Configuration files are updated to use the new option and to
signal the identity matrix in the case of direct GBR coding.