Put a TextureOptions and NoiseParams in the ShadingContext, rather than alloca

Put a TextureOptions and NoiseParams 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. Hopefully, this provides a workaround that avoids the LLVM bugs.