Add note regarding the fact that 4:4:0 lacks full SIMD support; Add an option for benchmarking 4:4:0 subsampling in tjbench.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@1009 632fc199-4ca6-4c93-a231-07263d6284db

turbojpeg.h

@@ -85,6 +85,8 @@ enum TJSAMP
   /**
    * 4:4:0 chrominance subsampling.  The JPEG or YUV image will contain one
    * chrominance component for every 1x2 block of pixels in the source image.
+   * Note that 4:4:0 subsampling is not fully accelerated in the libjpeg-turbo
+   * implementation of TurboJPEG.
    */
   TJSAMP_440
 };