Propagate origin displacements of child structures to parent

[kostmo]

Builds upon #2127 to fix the remaining issues with #1826.

If a structure incorporates sub-placements entailing northwesterly offsets, its "coordinate origin" will be shifted relative to the top-left cell in the grid. This updated coordinate origin should be propagated to parent structures for use when placing it. This includes placement of the main "area" onto the toplevel world map. This is essential when composing a large scene that needs to line up with features generated by the DSL.

Another bug fixed in this PR involved incorrect "area" computation within sibling placements when both a "northward" and "westward" offset were used; existing tests only covered each of these directions separately.

Changes in this PR

• Refactoring for readability
• Improved naming
  • Fixed typo padSouthwest -> padNorthwest
• Export some functions for unit tests
• Utilize propagated coordinate offset in WorldDescription

Testing

Unit tests

scripts/test/run-tests.sh swarm-unit --test-options '--pattern "Overlay"'

Scenarios

scripts/play.sh -i data/scenarios/Testing/1780-structure-merge-expansion/coordinate-offset-propagation.yaml --hide-goal

Before	After

scripts/play.sh -i data/scenarios/Testing/1780-structure-merge-expansion/simultaneous-north-and-west-offset.yaml --hide-goal

Before	After