Add gray16 and rgb48 png encoding regression tests.

tests/lavf-regression.sh

@@ -155,6 +155,8 @@ fi
 
 if [ -n "$do_png" ] ; then
 do_image_formats png
+do_image_formats png "-pix_fmt gray16be"
+do_image_formats png "-pix_fmt rgb48be"
 fi
 
 if [ -n "$do_bmp" ] ; then

tests/ref/lavf/png

 c162094e51dc1a3203de43e496086dfd *./tests/data/images/png/02.png
 ./tests/data/images/png/%02d.png CRC=0x6da01946
 248612 ./tests/data/images/png/02.png
+63805114b3723490833789277246276f *./tests/data/images/png/02.png
+./tests/data/images/png/%02d.png CRC=0x831a2963
+41666 ./tests/data/images/png/02.png
+b4c1f0b706a296ba6e791d436757dc5f *./tests/data/images/png/02.png
+./tests/data/images/png/%02d.png CRC=0x5984c023
+511879 ./tests/data/images/png/02.png