Fix compilation with GCC 4.2.1 (OS X 10.7).

GCC was preferring to cast float4 to Dual2<float4> and call the locally defined versions of shuffle() etc, rather than call the correct OIIO::simd versions. Fully qualifying the calls with the namespace fixes that.

This doesn't seem ideal, as it's quite verbose, and also prone to being rebroken when new code is introduced. I did try removing the `using namespace OIIO::simd` to prevent the latter problem, but then even more verbosity would be required. Perhaps removing the `using` and replacing it with a short namespace alias would be preferred?

I have mailed a completed CLA to opensource@imageworks.com.