h264parse: Allow parse to update framerate from parsed value
Depending on stream, upstream framerate (e.g., set by demuxer) might be incorrect but it can be corrected by h264parse using parsed value. And in case of framerate sensitive scenario such as timecode or closed caption handling, parsed framerate can be preferred over upstream one. Adding new property "framerate-override-mode" for user to be able to select upstream-framerate preference behavior.