caption_frame_decode: rework API

[MathieuDuponchelle]

Fixes #59

• pop_on mode requires incrementing the frame timestamp until end_of_caption is encountered.

• caption_frame_decode now always updates the timestamp of the frame when the timestamp parameter != -1. This requires that callers only pass a valid timestamp when a new one is encountered, for example with SCC the timestamp at the start of the cue, then -1 until the next new timestamp.

• A new enum member is added for the return value, LIBCAPTION_CLEAR. It allows the caller to determine that closed captions should not be displayed anymore, in order to finish the previous cue earlier than the start of the next cue.

[MathieuDuponchelle]

wget https://publish.dvlabs.com/democracynow/360/dn2018-1217.mp4

Before:

orig.txt

After:

new.txt

Rename to .srt if needed, github was refusing that extension

[MathieuDuponchelle]

ping @szatmary :)