swscale/utils: Fix color range of gray16

Improves rgb -> gray16 conversion

Fixes Ticket3422

The pam and png output files look visually similar, in both cases the
dynamics increase to 0x0 -> 0xfffb.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/utils.c

@@ -1055,6 +1055,8 @@ static int handle_jpeg(enum AVPixelFormat *format)
         *format = AV_PIX_FMT_YUV440P;
         return 1;
     case AV_PIX_FMT_GRAY8:
+    case AV_PIX_FMT_GRAY16LE:
+    case AV_PIX_FMT_GRAY16BE:
         return 1;
     default:
         return 0;