fix(core): shader-based globe occlusion for billboard layers (IconLayer, TextLayer)

[Copilot]

Remove ArcLayer

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[charlieforward9]

@copilot partial layer depthtesting?

[Copilot]

@copilot partial layer depthtesting?

Depth testing doesn't work correctly for billboard layers (IconLayer, TextLayer) even when enabled per-layer. The reason: billboard vertices use screen-space pixel offsets via project_pixel_size_to_clipspace — the anchor point may be behind the globe, but its screen-space-offset fragments end up at a depth value that passes the depth test since there's nothing in the depth buffer at those pixels. That's why shader-based occlusion at the anchor position is needed for billboards.

For non-billboard geometry like ArcLayer, regular depth testing does work — in interleaved mode the basemap provides a globe-shaped depth buffer that correctly occludes arcs going behind the globe, which is why ArcLayer was removed from scope.