Make Flashback's sky override option work with iris shaders.

Notice:

Compared to version 1.0.0, which covered the sky by rendering a box, version 2.0.0 directly disables the functions of

WorldRenderingPhase.SKY, WorldRenderingPhase.CUSTOM_SKY, WorldRenderingPhase.CLOUDS, WorldRenderingPhase.VOID, WorldRenderingPhase.SUN, WorldRenderingPhase.STARS, WorldRenderingPhase.MOON, and WorldRenderingPhase.SUNSET,

which should yield a better result. However, you must manually disable clouds in your shader settings. In version 1.0.0, the clouds only appeared to be covered, but in reality, they were simply beyond the boundaries of the green-screen box.

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