Buffers should actually be cleared before running operations on them,
but this doesn't work for some reason.
Note also that the sunbeams node can show some creases and hard aliasing
when the source point is close to a bright area with strong gradient.
To fix this a better filtering algorithm, dithering or ray sampling
would need to be implemented. In the meantime simply blurring the
sunbeams result a bit should help (or simply avoid putting the source on
a bright spot).
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