[BUG] DPX from "Resolve Live" mangled conversion

**To Reproduce**
Use oiiotool (2.5.4.0, or really any version AFAICT) to convert this [DPX file](https://github.com/AcademySoftwareFoundation/OpenImageIO/files/13207430/RL01_00411952.dpx.zip) to a jpg or any other format. Notably OIIO thinks this is a single-channel image. Resolve indicates that it is at least a RGB image. This DPX was captured in a Resolve Live session.

```
oiiotool -i RL01_00411952.dpx -o test.jpg
```

**Expected behavior**
An RGB image (Screenshot from Resolve):

**Actual behavior**
oiiotool creates a greyscale JPEG

**Platform information:**
- OIIO branch/version: 2.5.4.0
- OS: macOS
- Homebrew installation

Start by reproducing the conversion with the attached DPX file using the documented oiiotool command, then trace how OIIO interprets the DPX channels. Done means the file is recognized as RGB and conversion to JPEG produces a color image rather than grayscale.

Written by the indexing model from the issue text.