Force I-frame on SCTE-35 breaks

[DenizUgur]

Added a new option for scte35dec that would copy the incoming packets to its output. During this operation, if a SCTE-35 splice point is encountered, that packet is marked to be encoded as an I-frame.

I wanted to use GF_PROP_PCK_CUE_START but that forces the encoder to reconfigure and during my tests libx264 entered into a reconfiguration loop.

• Test with more content
• Decide on the option name
• Find out how to work with GF_PROP_PCK_CUE_START
• Not all events trigger IDR, need to handle that scenario
• unit tests
• test suite

[DenizUgur]

@jeanlf any idea why GF_PROP_PCK_CUE_START puts ff_enc into reconfiguration loop? From what I can tell, the encoder is trying to consume the same packet repeatedly and that causes this loop. There might be other stuff as well.

[DenizUgur]

Last commit fixed the encoder reconfiguration issue. It can be cherry-picked from this PR if it's satisfactory.

[rbouqueau]

LGTM