* There is now a bool setting to turn off depth writing and testing for
RenderableSpheres (useful for background images).
* There is also an Option setting to choose blending mode, similar to
the setting that RenderablModel has.
* NOTE: The default behaviour for RenderableSphere have now changed.
Before it was always additive blending that was used but now it is
default blending that is used by default. If additive blending is
wanted, then use the option setting and select it instead.
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