Images data validation

Right now, Validator checks only that image dimensions are powers-of-two.

Next checks:
- [ ] Detect when number of channels doesn't seem to match material usage (e.g., pure grayscale image used for Metal-Roughness texture).
- [ ] Detect when image defines custom colorspace/gamma/intent.
- [ ] Detect when image uses non-square pixels.
- [ ] Detect EXIF Orientation flag in JPEG images.

Thoughts?

Start by reading Validator's existing image-dimension power-of-two checks. Determine how image channels, colorspace metadata, pixel aspect ratio, and JPEG EXIF Orientation are exposed, then verify that validation covers each listed case without incorrectly flagging valid images.

Written by the indexing model from the issue text.