Basic SIMD support in the form of float4, int4, and mask4 classes
that take advantage of SSE if available.

This is a first stab at it, and will be augmented in the future
with additional functionality, additional architectures (AVX and Neon
come to mind), etc. But this is enough to get started.
Bump version with SIMD
make/CMake support for SIMD building modes (e.g. 'make USE_SIMD=sse4.1')
Bump SPI minimal SSE level to 4.1 on spinux, 4.2 on OSX
ImageBuf and ImageCache: pad buffers by an extra SIMD width so that SIMD
reads don't overstep the allocation sizes.
SIMD instruction speedups for texture lookups

This is a big overhaul of the internals of TextureSystem to make as much
as possible use SSE (and in future updates, other SIMD instruction
sets).  We're seeing 30-60% reduction in time of the texture lookups
themselves, depending on the SSE compatibility level, number of channels
looked up, whether texture derivatives are requested, etc.  Call it
close to a 2x speedup in general. But of course, how much this will
translate into speedup of an app using TextureSystem will depend a lot
on the original percentage of the runtime of the app that was spent
doing the texture math.

This is a first stab, there is more to return to in the future. For
example, we have only SSE-optimized texture(), but really not
texture3d() or environment(). There's probably even more we can squeeze
out of texture() if we try.
Overhaul TextureSystem API -- eliminate nchannels and deriv fields from
TextureOpt, in favor of passing them explicitly to the various texture
API functions. This is all in support of the SIMD internals changes.
CHANGES