Add independent audio pitch and playback speed controls (#1056)

[rajeshgk]

Summary

Implements independent audio pitch and playback-speed controls in libopenshot, addressing issue #1056.

The implementation introduces optional Rubber Band processing to support time stretching and pitch shifting independently.

Changes

• Add pitch and preserve_pitch keyframe controls to Clip.
• Add a PitchProcessor wrapper around Rubber Band.
• Integrate pitch-preserving audio processing with clip time mapping.
• Make Rubber Band an optional build dependency.
• Preserve existing audio processing behavior when Rubber Band is unavailable.
• Maintain backward compatibility with existing clip JSON.
• Add regression tests covering pitch preservation and octave adjustments at different playback speeds.

Validation

Rubber Band enabled

• 48/48 Clip tests passed.
• Verified pitch preservation at 0.5× and 2× playback speeds.
• Verified +12 semitone pitch adjustment at 0.5× speed.
• Verified −12 semitone pitch adjustment at 2× speed.

Rubber Band disabled

• 47/47 Clip tests passed.
• Confirmed compatibility with the legacy audio-processing configuration.

Both configurations passed with no test failures.

Compatibility

Rubber Band is optional. Existing builds without Rubber Band continue using the legacy audio-processing path.

Existing clip JSON without the new properties retains neutral pitch defaults.

Related issue

Addresses #1056.