Some SIMD odds and ends

1. Address SSE3 vs SSSE3.

   Brecht points out that we set OIIO_SIMD_SSE to 3 when detecting `__SSE3__`, but actually, some instructions may require SSSE3. Oy.

   My assumption is that there are only a few old (only 32 bit?) architectures that are SSE3 but *not* SSSE3, and that it's no big deal for those oddballs to get SSE2. So my proposed solution is to set OIIO_SIMD_SSE to 3 when we see `__SSSE3__`. Simple, and will not affect the vast majority of performance-needing users who are long past SSE3 and have SSE4.x or AVX.

2. unary - operator should have been const all along, now fixed.

3. Make a TypeDesc::TypeFloat4 for convenience.