Port RGB-to-Grayscale color transform from jpeg-8d

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@885 632fc199-4ca6-4c93-a231-07263d6284db

libjpeg.txt

@@ -1,7 +1,7 @@
 USING THE IJG JPEG LIBRARY
 
 This file was part of the Independent JPEG Group's software:
-Copyright (C) 1994-2009, Thomas G. Lane, Guido Vollbeding.
+Copyright (C) 1994-2011, Thomas G. Lane, Guido Vollbeding.
 Modifications:
 Copyright (C) 2010, D. R. Commander.
 For conditions of distribution and use, see the accompanying README file.
@@ -1279,8 +1279,9 @@ jpeg_read_header's guess by setting jpeg_color_space.  jpeg_read_header also
 selects a default output color space based on (its guess of) jpeg_color_space;
 set out_color_space to override this.  Again, you must select a supported
 transformation.  jdcolor.c currently supports
-	YCbCr => GRAYSCALE
 	YCbCr => RGB
+	YCbCr => GRAYSCALE
+	RGB => GRAYSCALE
 	GRAYSCALE => RGB
 	YCCK => CMYK
 as well as the null transforms.  (Since GRAYSCALE=>RGB is provided, an