h261: Signal freeze picture release for intra frames

Freeze picture release should be set to 1 when we're responding to a
fast update request. For simplicity we set it for all intra frames,
including those that starts a GOP.

Fixes issue where Tandberg MXP1700 does not recover from packet loss
state since it's waiting for the freeze picture relase indication.

Bug-Id: 873
CC: libav-stable@libav.org

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>