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Emits when a layer is refused, fails to draw, or is given up on. Observe with onLayerError.observe(this) { error -> ... }; for coroutine / flow access see onLayerErrorValue.
A failing layer never terminates the process (see reportLayerError), so without this callback a layer that could not be added or could not compile its shaders was silent in a release build - indistinguishable from one that had no data to draw. See MapsGLLayerError for what each MapsGLLayerError.Reason means.