With this helper API, filters that used the first framesync
helper API can easily be changed to use the new and more
extensible design for filters with a single activate() callback.

framesync2 will be the base for the version using activate.
Most of the logic will be the same, but the code cannot be shared.
Copying the file initially without change will make the diff
easier to read.

Fix an infinite loop in forward_status_change().

It has no longer any effect.

Signed-off-by: NPaul B Mahol <onemda@gmail.com>

Compared to dualinput, this API can handle more than two
inputs and can generate frames synchronized to any or all
input streams.