lavfi/curves: support preset shorthand.
mpegtsenc: Only test the first frame for missing h264_mp4toannexb filter.

Many video players accept broken frames in a transport stream, so there
is no reason to abort remuxing when encountering one, just print a
warning instead.

Fixes ticket #1758.
Only test the first frame for missing aac_adtstoasc bistream filter.

Many players ignore broken aac frames, so don't abort mov or flv
muxing when encountering one, just print a warning instead.

Fixes ticket #2380.