refactor(video): move encoder declarations to header

[ReenigneArcher]

Description

This PR aims to simplify creation of encoder fixtures for unit testing (#1603).

Screenshot

Issues Fixed or Closed

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch
must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

[ReenigneArcher]

Docs build fails due to duplicate C++ declaration. breathe-doc/breathe#972

[kentyman23]

My using directive comment is the only one that might warrant a change.

[ReenigneArcher]

This was refactored slightly to avoid using namespace std::literal; in the header.

[cgutman]

Is something else pulling this in for the swscale APIs below?

[ReenigneArcher]

Probably swscale from the video.h

[ReenigneArcher]

I can leave it in video.cpp as well, if that's preferred.

[ReenigneArcher]

Converted to draft as there are a probably a few declarations that would need to be moved to the header in order to use for unit testing.

At least the following, but maybe more.

  bool
  validate_encoder(encoder_t &encoder, bool expect_failure);

[ReenigneArcher]

I have a working H264 software encoder fixture, so I think this is ready

Edit: I now have parametrized encoder fixtures, unavailable encoders are skipped automatically except the software encoder which is set to fail since it "should" be available in all cases.