Sightline Receiver Program
interface SightlineReceiverProgram
Contract for shader programs that splice in SightlineReceiverGlsl. Identity-keyed cache: earth.worldwind.draw.DrawableSightline allocates a fresh SightlineState every frame, so receivers compare the new state instance against the last one they uploaded and skip the work when they match (covers the within-frame "second program-bind sees the same state" case).