The x86/x86-64 SIMD extensions were originally designed to accommodate changing the...

The x86/x86-64 SIMD extensions were originally designed to accommodate changing the value of RGB_*, but this apparently broke when RGB-to-gray colorspace conversion was accelerated.  Further, the ARM NEON extensions have always assumed that JCS_RGB behaves identically to JCS_EXT_RGB.  Rather than fix these issues, it makes more sense to just stop claiming that we support changing the values of RGB_*, since doing so is no longer necessary.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/branches/1.3.x@1298 632fc199-4ca6-4c93-a231-07263d6284db