fix #201 Division by zero in FScene::computeLightRanges (#217)
fix #201 Division by zero in FScene::computeLightRanges (#217)

SIMD code is accessing uninitialized elements,
so make sure they are initialized enough to avoid
divide-by-zero.

We were also processing the directional light in
computeLightRanges(), which always has a zero
radius.
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