Fix non-deterministic gvar compilation

[anthrotype]

Fixes #1873

ensure_composite_defined_at_component_locations and ensure_component_has_consistent_layers incrementally inserted interpolated instances into a glyph's source map, mutating the VariationModel between iterations. Combined with non-deterministic HashMap iteration order, this produced different interpolation results across builds.

The fix is to collect all missing locations first, interpolate all from the unmodified source map, and insert them at the end after the loop. Each interpolation now sees the same VariationModel regardless of processing order.

I derived two reproducers from Georama's topology (4 corner masters + 5 intermediates including an interior off-axis location) that verify each interpolated instance returned from these functions matches an independent interpolate_instance call on the original glyph.

Note that because of the non-determinism, the test may pass the first time the CI runs if the random iteration order happens to be the lucky one.

I also confirmed that Georama deterministically builds the same gvar after the fix.

[anthrotype]

gladly both failed at first shot:

https://github.com/googlefonts/fontc/actions/runs/24506123258/job/71624978566?pr=1960#step:5:244

---- glyph::tests::component_consistent_layers_independent_of_insertion_order stdout ----

thread 'glyph::tests::component_consistent_layers_independent_of_insertion_order' (4068) panicked at fontir/src/glyph.rs:2560:13:
assertion `left == right` failed: instance at Normalized {wdth: 0.40, wght: 0.50} differs from independent interpolation on the original component
  left: GlyphInstance { width: 0.0, height: None, vertical_origin: None, contours: [BezPath([MoveTo((-384.1, 740.9000000000001)), LineTo((-269.40000000000003, 740.9000000000001)), LineTo((-347.40000000000003, 912.4000000000001)), LineTo((-493.8000000000001, 912.4000000000001)), ClosePath])], components: [] }
 right: GlyphInstance { width: 0.0, height: None, vertical_origin: None, contours: [BezPath([MoveTo((-399.90000000000003, 744.7)), LineTo((-300.6, 744.7)), LineTo((-399.6, 925.2)), LineTo((-525.6000000000001, 925.2)), ClosePath])], components: [] }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- glyph::tests::interpolated_instances_depend_only_on_original_sources stdout ----

thread 'glyph::tests::interpolated_instances_depend_only_on_original_sources' (4097) panicked at fontir/src/glyph.rs:2459:13:
assertion `left == right` failed: instance at Normalized {wdth: 0.40, wght: 0.50} differs from independent interpolation on the original glyph
  left: GlyphInstance { width: 0.0, height: None, vertical_origin: None, contours: [], components: [Component { base: uni0306, transform: Affine([1.0, 0.0, 0.0, 1.0, 0.0, 0.0]), anchor: None }, Component { base: gravecomb.case, transform: Affine([0.95, 0.0, 0.0, 0.95, 51.69999999999999, 72.2]), anchor: None }] }
 right: GlyphInstance { width: 0.0, height: None, vertical_origin: None, contours: [], components: [Component { base: uni0306, transform: Affine([1.0, 0.0, 0.0, 1.0, 0.0, 0.0]), anchor: None }, Component { base: gravecomb.case, transform: Affine([0.95, 0.0, 0.0, 0.95, 51.699999999999996, 72.2]), anchor: None }] }

[cmyr]

good catch, this has been annoying me :)