feat: variable line-offset using line-progress

[chuhlomin]

Adds support for using line-progress expressions in line-offset to create variable line offsets that change along a line's length.

This enables use cases like subway/transit maps with lines that merge and diverge.

Note: Most of this implementation was generated with assistance from Claude Sonnet 4.5. This is a feature I've wanted for a side project for about 2 years, and I couldn't have completed it without AI assistance.

Fixes #12729, #10374

Implementation

• Shader changes: Added VARIABLE_LINE_OFFSET define and extended vertex attribute usage
• Bucket evaluation: Added evaluation of variable offset values alongside existing variable width support
• Feature interaction: Properly handles cases where both VARIABLE_LINE_WIDTH and VARIABLE_LINE_OFFSET are active
• Style spec: Updated line-progress documentation to include line-offset property

The performance impact should be negligible for typical use cases.

Example Usage

paint: {
  'line-offset': [
    'interpolate',
    ['linear'],
    ['line-progress'],
    0, -10,  // Start offset
    1, 10    // End offset
  ]
}

Demo

See debug/variable-line-offset-demo.html for a working example showing a line with offset varying from -10px to +10px along its length.

@mapbox/map-design-team, @mapbox/static-apis this PR includes style spec API
@mapbox/gl-native this PR includes shader changes

[CLAassistant]

All committers have signed the CLA.