Ref H.261 recommendation section 4.2.1.3, setting the still image flag
to 1 disables still image mode. Some decoders require this in order to
decode the bitstream as normal video.

Fixes H.261 calls to Cisco E20.

Also, reserved (aka spare) bits should be set to 1 unless specified
otherwise.

Bug-Id: 872
CC: libav-stable@libav.org
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>