h264parse: Add flag to not reset state if a SPS is received after a PPS (needed for some cameras).

I have an USB camera streaming H264. The stream was not correctly handled by h264parse because the camera send regularly Sequence Packet Set while just sending Picture Parameter Set at startup. In that case h264parse reset its internal state and consequently is not able to parse the stream.

The behavior of my camera is probably very specific so this fix is activated by a new option "reset-on-sps". This option is defaulted to true (old behavior), setting it to false prevents the state to be reseted when receiving a SPS.

This issue is also described on bug #571 (closed) and should fix it also (same fix but with an option to enable it).