Since their introduction, generated UDIM images/tiles would not be
loaded in Cycles. In relative recent history 72ab6faf5d added the
ability to track which tiles were still marked as being "generated" and
unsaved. However, that check was never implemented into the Cycles code.
With this PR, these two additional scenarios should now work:
- Properly load pixels if all tiles are generated
- Properly load pixels if a UDIM image has been loaded from a file and
new generated tiles have been added but not saved yet etc.
Pull Request: https://projects.blender.org/blender/blender/pulls/127673