Web Viewer: Add support for drag-and-drop of materials with image references

## Issue

If you drag-drop a MTLX file with image dependencies the image dependencies cannot be found to be bound.
Seems to be the case for

1. Image files in same folder as MTLX file
2. Image files in sub-folders of MTLX file
3. Image files in sibling or parent folders

Perhaps this is due to the usage of relative file paths, though would think that 1 would work.

## Diagnosis

Found while loading and examining the console log in a browser.

Reproduce the drag-and-drop flow in the Web Viewer with an MTLX file and image references in the same, child, sibling, and parent folders. Start by examining the browser console log and trace how relative image paths are resolved; done means all listed dependency locations bind successfully.

Written by the indexing model from the issue text.