RendererServices API overhaul -- all the texture functions have been
slightly changed to explicitly pass nchannels and the derivative pointers
rather than embedding those in the TextureOpt struct. This goes along with
an OIIO-side change that altered the TextureSystem API in a similar way and
eliminated those fields from TextureOpt. This was part of a big overhaul
aimed at SSE-izing the TextureSystem internals.
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.
simd Make/cmake
Bump SPI minimal SSE level to 4.1 on spinux, 4.2 on OSX
Merge pull request #410 from lgritz/lg-simd

Use SIMD in texture system