USD material import improvements.

Refactored get_udim_tiles() based on review by
Sybren and Jesse.

USD material import improvements.

Per review by Sybren, added find_existing_material()
utility function and comments.

USD material import improvements.

Per suggestions from Jesse and Sybren in their
reviews.

USD import: fixed material reader comments.

Fixed typos in material reader code, as noted in
the patch review comments.

This review fixes USD Preview Surface import bugs reported in T90535 (missing materials in Animal Logic's ALab).

Differential Revision: https://developer.blender.org/D13297

USD Preview Surface material import improvements.

Updates to address issues importing USD Preview Surface
materials in Animal Logic's ALab scene, as described in
T90535:

Added support for importing UDIM textures.

Added 'Material Name Collision' USD import menu option,
to specify the behavior when USD materials in different
namespaces have the same name. The options are

Modify: Create a unique name for the imported material.
Skip: Keep the existing material and discard the imported material.

Previously, the default behavior was Skip. This was causing an
issue in the Alab scene, where dozens of different USD materials
all have the same name 'usdpreviewsurface1', so that only one
instance of these materials would be imported.

Finally, if no materials with purpose "render" are assigned to
the USD primitive, the importer will now fall back on converting
assigned materials with purpose "preview".