Removes the artificial requirement that UDIM tile sets start at 1001. Blender was already capable of handling sparse tile sets (non-contiguous tiles) so the restriction around starting at 1001 was unnecessary in general. This required fixing a few UDIM-related python bugs around manually updating the `tile_number` field on images as well. See the differential for details. No script changes are necessary but they will now work, correctly, in many more cases. Differential Revision: https://developer.blender.org/D11859