Blender Git Loki

USD IO: material conversion improvements.

Updated the USD Preview Surface texture node
import code to handle UDIM tiles that don't
start a 1001. Performed miscellaneous cleanup
to make code more robust.

Fixed logic for the MDL material fallback behavior
to import the USD Preview Surface shaders only if
the material has no MDL shaders. I.e., it will not
load preview surface as a fallback if an MDL exists
but failed to load for some reason. This is much
more useful for debugging failures and also gives
the user an opportunity to fix a partially successful
MDL import. This refactor also fixes a significant
bug where the fallback would be used even if the MDL
import succeeds. Refactored the report_notification()
utility function in the UMM conversion code to return
more meaningful results.

Added logic to generate file names for packed texture
assets when exporting USD Preview Surface shaders.
Previously, such asset paths were left empty and were
omitted from the export.

Updated the UsdUVTexture shader conversion code to
handle the case where the file input has a connected
source, which may happen if this input is overridden
by an input on the parent material.

Made the logic for determining the color
space for texture assets when collecting UMM
source data more robust by handling the case
where a connected source input has no color
space specified. The fix is to also query
the shader's input attribute for this data.