Missing USD Shadow API and Shaping API support for lights

[bbias]

Problem

The current light implementation lacks support for USD's Shadow API and Shaping API, which are standard parts of the UsdLux schema.

Missing Shadow API Attributes

All light types should support:

• inputs:shadow:enable (bool)
• inputs:shadow:color (color3f)
• inputs:shadow:distance (float)
• inputs:shadow:falloff (float)
• inputs:shadow:falloffGamma (float)

Missing Shaping API Attributes

Boundable lights (SphereLight, RectLight, DiskLight, CylinderLight) should support:

• inputs:shaping:focus (float)
• inputs:shaping:focusTint (color3f)
• inputs:shaping:cone:angle (float)
• inputs:shaping:cone:softness (float)

Current Behavior

• Shadow and shaping attributes are ignored during parsing
• USD files with these attributes lose important lighting information

Expected Behavior

• All shadow attributes should be parsed and stored for all light types
• Shaping attributes should be parsed only for appropriate boundable light types
• Proper USD specification compliance

Impact

Lighting information is lost when loading USD files, affecting rendering accuracy and compatibility.

[syoyo]

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