MixPresentationOBU syntax/semantics simplification

[cconcolato]

In the context of #759, I found it hard to navigate the spec around Mix Presentation OBU parameter definitions (element and output gains). IMHO, it could be simplified without changing the semantics or bitstreams, as follows:

The Mix Presentation OBU uses:

ElementMixConfig element_mix_config;

but ElementMixConfig is simply

class ElementMixConfig() {
  MixGainParamDefinition mix_gain;
}

My suggestion would be to remove the use of ElementMixConfig and simply use the following in the MixPresentationOBU

 MixGainParamDefinition element_mix_gain;

It means also moving the semantics of mix_gain from section 3.7.4 to section 3.7, merging it with the semantics of element_mix_config

Same for output_mix_config. The syntax in MixPresentationOBU should simply have:

MixGainParamDefinition output_mix_gain;

instead of

OutputMixConfig output_mix_config;

Section 3.7.5 can then be removed and merged with the semantics of output_mix_config (which would become output_mix_gain).

[cconcolato]

Similarly, the annotation syntax description could be simplified, instead of defining a class with a single field:

class MixPresentationElementAnnotations() {
  string audio_element_friendly_label;
}

or

class MixPresentationAnnotations() {
  string mix_presentation_friendly_label;
}

The MixPresentationOBU could simply contain:

  for (i = 0; i < count_label; i++) {
    string presentation_annotations;
  }

and

      for (i = 0; i < count_label; i++) {
        string element_annotations;
      }

In this case, I think also the use of arrays would simplify further the syntax.

string language_labels[count_label];
string localized_presentation_annotations[count_label];
...
string localized_element_annotations[count_label];

[tdaede]

We should look if we expect these to be updated right away in the version 2. If so we can leave them as is, if not we can simplify them.