Mark Tucker

Fix UDIM path resolution for scene index plugins

3

### Description of Change(s) Implement a specialization of UsdImagingDataSourceAttribute that properly resolves UDIM relative paths. Note that I have not created a unit test, or run the unit tests yet....

Composition issue when specifying an invalid defaultPrim

1

Unzip the attached empty.usda and prims.usda: [defaultPrimIssue.zip](https://github.com/user-attachments/files/23062164/defaultPrimIssue.zip) 1. Open `usdview empty.usda` 2. In the interpreter window, run: ``` s = usdviewApi.stage l = s.GetRootLayer() with Sdf.ChangeBlock(): l.subLayerPaths = ["prims.usda"] l.defaultPrim...