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
400cb25fc7