perf(Prop3D): only recompute the matrix when the transform changes

[cpruijsen]

Context

vtkActor.onModified triggers computeMatrix() on every modified() event, even when only the visibility or pickability changes. The matrix is already recomputed lazily on access, so the eager recompute is wasted work.

fix #3378

Results

Before: setVisibility(), setPickable() or a vtkProperty change each rebuilt the actor's 4x4 matrix, once eagerly inside onModified and again on the next getMatrix()/computeBounds()/per-frame computeMatrix() call from the OpenGL/WebGPU view nodes, because the staleness guard compared the prop's global mtime.

After: the matrix is rebuilt only when a transform input (origin, position, scale, orientation/rotation, userMatrix) changes. isIdentity is still refreshed on transform changes via the existing onModified subscription, which now early-returns after a single timestamp comparison for unrelated modifications.

Verified by a new regression test in Sources/Rendering/Core/Prop3D/test/testComputeMatrix.js: after setVisibility(false), setPickable(false) and a property opacity change, the matrix's mtime is unchanged across getMatrix()/computeMatrix(); after setPosition/setOrigin/setScale/rotateZ/setOrientation it advances and the matrix content is correct.

Note on approach: the staleness check in computeMatrix() was narrowed to a dedicated transform-inputs mtime rather than only removing the onModified(computeMatrix) subscription. Removing the subscription alone would leave the lazy path recomputing after unrelated modified() calls (the guard keyed on the global mtime), and getIsIdentity() would go stale between a transform change and the next access. The subscription can still be removed if reviewers prefer.

Changes

• In Sources/Rendering/Core/Prop3D/index.js, vtkProp3D.computeMatrix() compares a new model.transformsMTime against model.matrixMTime instead of the prop's global getMTime().
• transformsMTime is bumped by setOrigin/setPosition/setScale (via _onXXXChanged model hooks, the same mechanism vtkImageData uses) and by addPosition, rotateX/Y/Z, rotateWXYZ, rotateQuaternion, setOrientation, setOrientationFromQuaternion, setUserMatrix.
• No public API added, deleted, or changed. vtkFollower.computeMatrix() is unchanged: its matrix depends on the camera, which the transform mtime deliberately does not cover.

PR Checklist

• GitHub Actions CI passed: semantic-release commit messages, lint, and tests
• Test coverage added: Sources/Rendering/Core/Prop3D/test/testComputeMatrix.js (fails without the fix: AssertionError: expected 10 to be 2)
• Documentation and TypeScript definitions are updated to match these changes (no public API change; no .d.ts update needed)