Add a "fatal" function for multi-output Map shaders #307

In moonray: lib/rendering/rndr/RenderContext.{h,cc} Add a "fatal" function analogous to fatalSampleBool for multi-output shaders (fatalSampleMulti?) and call mSceneContext->setFatalSampleFuncMulti() to set it on the context.

A trivial implementation of this would be to get the size of the output buffer and zero it while a more advanced one would iterate through the outputs and set an appropriate error value for each type. This may be awfully expensive though. Is there someplace we could cache an error value?