Scope face fill triangle map to selected UDIM

[Loggableim]

Summary

• use the selected paint object for the face-fill triangle map when a layer or object filter is active
• retain merged-mesh behavior when no filter is active
• add a focused regression check

Root cause

The triangle map was always generated from the merged mesh. Overlapping UVs from another UDIM could therefore overwrite the selected tile's face identity.

Validation

• python paint/tests/test_issue_2081.py
• Local native project generation is currently blocked by the bundled amake stack-overflow error; GitHub platform builds will provide compile validation.

Fixes #2081