Missing RectLight support in USD parser

[bbias]

Problem

The USD parser currently does not support RectLight primitives, which are part of the standard UsdLux schema.

Current Behavior

• RectLight primitives are not registered in the parser
• Files containing RectLight fail to parse or are treated as unknown primitives
• Missing from composition system reconstruction

Expected Behavior

• RectLight should be fully supported like other light types (SphereLight, DiskLight, etc.)
• Should parse RectLight attributes: width, height, intensity, color, etc.
• Should be included in composition system

Impact

Users cannot load USD files containing RectLight primitives, limiting compatibility with standard USD content.

[syoyo]

PR has been merged. I think this issue is ready to close.