Custom camera attributes aren't considered in UsdImagingCameraAdapter::TrackVariability

[robpieke]

Description of Issue

If you apply a custom schema to a camera (for example to attach a background image) and populate the custom attribute(s) with time-sampled data, Hydra doesn't trigger the appropriate syncing on frame changes. This is due to UsdImagingCameraAdapter::TrackVariability having a hard-coded list of attributes to consider.

In contrast, UsdImagingLightAdapter::TrackVariability iterates over all attributes.

PR for consideration inbound ... basically just copying what the lightAdapter does, at the end of the cameraAdapter :)

[jilliene]

Filed as internal issue #USD-7258

[sunyab]

PR #1798 was merged and released with v22.08. Closing, thanks!