OSL-side changes corresponding to SIMD overhaul of TextureSystem.
OSL-side changes corresponding to SIMD overhaul of TextureSystem.

TextureSystem has eliminated the nchannels and derivs field from
TextureOpt, instead passing those explicitly to the texture API functions.
We change here to match. Also, we make use of the new OIIO::simd::float4
class, to exploit SSE and other instruction-level SIMD in the glue code
surrounding the texture calls.

rlmh/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,imageworks/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,imageworks/OpenShadingLanguage,yaroslav-tarasov/OpenShadingLanguage,johnhaddon/OpenShadingLanguage,sjparker/OpenShadingLanguage,gzorin/OpenShadingLanguage,sjparker/OpenShadingLanguage,rlmh/OpenShadingLanguage,lgritz/OpenShadingLanguage,rlmh/OpenShadingLanguage,svenstaro/OpenShadingLanguage,imageworks/OpenShadingLanguage,mcanthony/OpenShadingLanguage,yaroslav-tarasov/OpenShadingLanguage,yaroslav-tarasov/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,mcanthony/OpenShadingLanguage,mcanthony/OpenShadingLanguage,brechtvl/OpenShadingLanguage,lgritz/OpenShadingLanguage,juvenal/OpenShadingLanguage,aconty/OpenShadingLanguage,imageworks/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,lgritz/OpenShadingLanguage,brechtvl/OpenShadingLanguage,mcanthony/OpenShadingLanguage,brechtvl/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,brechtvl/OpenShadingLanguage,lgritz/OpenShadingLanguage,johnhaddon/OpenShadingLanguage,juvenal/OpenShadingLanguage,brechtvl/OpenShadingLanguage,johnhaddon/OpenShadingLanguage,gzorin/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,gzorin/OpenShadingLanguage,gzorin/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,svenstaro/OpenShadingLanguage,svenstaro/OpenShadingLanguage,juvenal/OpenShadingLanguage,sjparker/OpenShadingLanguage,aconty/OpenShadingLanguage,rlmh/OpenShadingLanguage,sjparker/OpenShadingLanguage,svenstaro/OpenShadingLanguage,aconty/OpenShadingLanguage,juvenal/OpenShadingLanguage,yaroslav-tarasov/OpenShadingLanguage,johnhaddon/OpenShadingLanguage,aconty/OpenShadingLanguage,imageworks/OpenShadingLanguage,aconty/OpenShadingLanguage,lgritz/OpenShadingLanguage,gzorin/OpenShadingLanguage,fpsunflower/OpenShadingLanguage