USD Import: format fixes.

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".