core: Better -nocorrect-pts pause and filter-added frames handling
Rewrite the -nocorrect-pts frame creation code. The new version always
updates the visible frame when seeking while pausing, and supports
filter-added frames. It can not time those properly though.
Now the handling of filter-added frames in MPlayer always uses the new
method independently of the value of correct-pts but MEncoder still
expects the old behavior. Add a global variable that is set under
MEncoder only and change the filters to choose behavior based on that
instead of the correct_pts option.