fix(TerrainExtension): Handle pick events in TerrainExtension & draped layers

[felixpalmer]

Followup to #9938 add support for TerrainExtension

Background

Getting 3D picking working with the TerrainExtension is a bit more involved as the shader flows are more complex. We need to handle:

• Hovering over draped layer, but obtain 3D position of terrain below
• Hovering over tile containing draped layer, but outside the draped layer
• Hovering over tile that doesn't contain draped layer

Screen.Recording.2026-02-25.at.16.39.48.mov

Change List

• Add _getDepthLayers to DeckPicker to explictly filter out correct layers when obtaining height
• Enhance terrain shader-module to allow depth picking of terrain position
• Fix blend modes so layer indices do not get lost when rendering via framebuffers

[chrisgervang]

Are there any caveats or downsides, or only net improvements from what you can tell?

The demo looks nice!

[chrisgervang]

Most of these changes are in the picking pass and I'm assuming won't change draw pass renders. Is this change perceptible to the user?

[felixpalmer]

No, except that picking works where it previously didn't

[felixpalmer]

any caveats or downsides

From my testing none, just an improvement

[coveralls]

coverage: 91.006% (-0.02%) from 91.026%
when pulling 0fb6205 on felix/rotate-pivot-3d-terrain
into 64af196 on master.