The code checked for the presence of more than one tile before
substituting the tile number into the filename, so if a one-tile
UDIM was used (or all but one tile were culled), the substitution
was skipped and as a result the file was not found.
With this change, the code explicitly tracks whether substitution
is required, avoiding this problem.
This also fixes another problem: The Environment texture never
does substitution since it doesn't support UDIMs, but before the
syncing code still inserted the placeholder into the filename if the
user selected a tiled background image.
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