Merge branch 'ijg.lossless' into dev

Lossless: Accommodate LJT colorspace/SIMD exts

In libjpeg-turbo, grayscale_convert() and null_convert() aren't the only
lossless color conversion algorithms.  We can also losslessly convert
RGB to and from any of the extended RGB colorspaces, and some platforms
have SIMD-accelerated null color conversion.

This commit also disallows RGB565 output in lossless mode, and it moves
the IsExtRGB() macro from cdjpeg.h to jpegint.h and repurposes it to
make jinit_color_converter() and jinit_color_deconverter() more
readable.

cdjpeg.h

@@ -164,6 +164,3 @@ EXTERN(FILE *) write_stdout(void);
 #ifndef EXIT_WARNING
 #define EXIT_WARNING  2
 #endif
-
-#define IsExtRGB(cs) \
-  (cs == JCS_RGB || (cs >= JCS_EXT_RGB && cs <= JCS_EXT_ARGB))