applySightlineReceiverUniforms

Binds the sightline depth cube on unit 5 and uploads the sightline uniforms into program. Skips the work when there's no active sightline this frame (or in pick mode), and clears applySightline so receivers fall through to "no tint".

Receiver programs feed camera-relative positions into emitSightlineVaryings (the same varying that drives the shadow receiver — see earth.worldwind.layer.shadow.ShadowReceiverGlsl for why raw ECEF float32 positions are unusable), so the world -> sightline-local matrix is re-based against the eye point here, in double precision, before upload.