Avoid double-indirection to get the RendererServices since it is available in ShaderGlobals
Put a TextureOptions, NoiseOptions, TraceOptions in the ShadingContext,
rather than generating code to alloca them on the stack when needed.
The alloca seems to tickle an LLVM bug that generates incorrect code on
64 bit Windows, making OSL as a whole unstable on that platform.
This change provides a workaround that avoids the LLVM bugs, while not
having any negative performance impact.