oggenc: Write stream metadata if available

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Revert "pnm: remove nonsense code"

Breaks decoding pgms with 256 < maxval < 65535.

Found-by: Carl Eugen Hoyos <cehoyos@ag.or.at>.

This reverts commit a0348d0966a81a66f3a1bf061576b24d5296b933.
Revert "Avoid overflows when reading pgm files with maxval != 255 and != 65535."

The patch worked on little endian because pgm decoding was broken
but it was not correct in any way.

This reverts commit 7651c0e49b1f6e66120ef32570a87e3f09224574.