FG-13915: Fix scale-invariant label picking

[niels-foxglove]

Changelog

Fixed scale-invariant billboard text labels not being pickable in the 3D panel.

Docs

None

Description

Fixes: FG-13915

Problem: Billboard text labels with scale_invariant=true (sizeAttenuation=false in three-text) were not clickable in the 3D panel's select mode. The Picker renders objects into a small offscreen target using camera.setViewOffset() to zoom into the area around the cursor, then reads back pixel colors to determine what was clicked. The sizeAttenuation=false vertex shader path added offsets in clip space (after projection), which produces incorrect positions when setViewOffset modifies the projection matrix.

Solution: Move the sizeAttenuation=false offset calculation from clip space to view space (before projection). Multiply by camera distance (-mvPosition.z) to compensate for perspective divide, maintaining constant pixel size. This approach is compatible with setViewOffset because the projection matrix is applied after the offset.

Before (clip-space offset, broken with setViewOffset):

three-text/src/LabelMaterial.ts

Lines 55 to 57 in 59edb17

	gl_Position = projectionMatrix * mvPosition;
	// Add position after projection to maintain constant pixel size

After (view-space offset, works with setViewOffset):

three-text/src/LabelMaterial.ts

Lines 55 to 60 in b7705f3

	// Add offset in view space before projection so that setViewOffset
	// (used by the Picker) works correctly. Multiplying by distance
	// compensates for the perspective divide, keeping constant pixel size.
	float dist = -mvPosition.z;
	mvPosition.xy += vertexPos * 2. * dist / resolution * scale;
	gl_Position = projectionMatrix * mvPosition;

Also renamed uCanvasSize uniform to resolution (Three.js convention) and changed its type from [0, 0] array to THREE.Vector2(1, 1) to use .copy() in onBeforeRender.

Testing: Verified with a test data server that publishes a 3D scene with both scale_invariant=true and scale_invariant=false labels. Confirmed:

• Before fix: only green (scale_invariant=false) labels pickable
• After fix: both green and yellow (scale_invariant=true) labels pickable
• Reverted to main and re-verified the bug returns

Note: This PR should be merged after #448 (WebGPU support) lands, then the app's Picker.ts resolution override (lines 297-302) should also be removed in a follow-up.

[linear]

FG-13915 Scale-invariant billboard labels are not pickable in 3D panel

[niels-foxglove]

I will come back to this. This has been a low priority bug that nobody has reported yet and this PR is a draft because its only an AI generated first attempt. I will create a real PR when I've done a review of the code myself to get a better understanding of what is going on.