on_end event for video.add_text method

[kessenza]

It would be very useful to have an on_end event handler on video.add_text to intercept when text goes out of the screen (with speed >0).
This is useful in particular when cycle=false so we can programmatically change text when one line of text ends.
Note that we can't achieve the same result with very long text and cycle=true because liquidsoap throws exception when it has to deal with text bigger than 1k chars (more or less).
The exception thrown is the following:

22/09/14 20:37:13 [clock.main:4] Source output.url failed while streaming: Failure("FT_Load_Char")!
2022/09/14 20:37:13 [clock.main:4] Raised by primitive operation at Ftlow.render_char in file "freetype/ftlow.ml", line 144, characters 2-60
2022/09/14 20:37:13 [clock.main:4] Called from Freetype.render_char in file "freetype/freetype.ml", line 108, characters 4-56
2022/09/14 20:37:13 [clock.main:4] Called from OFreetype.draw_gen in file "freetype/oFreetype.ml", line 71, characters 21-59
2022/09/14 20:37:13 [clock.main:4] Called from Video_text_camlimages.render_text in file "decoder/text/video_text_camlimages.ml", line 49, characters 4-215
2022/09/14 20:37:13 [clock.main:4] Called from Video_text.text#render_text in file "sources/video_text.ml", line 36, characters 33-74
2022/09/14 20:37:13 [clock.main:4] Called from Video_text.text#synthesize in file "sources/video_text.ml", line 61, characters 8-33
2022/09/14 20:37:13 [clock.main:4] Called from Synthesized.source#get_frame in file "sources/synthesized.ml", line 59, characters 8-37
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#instrumented_get_frame in file "source.ml", line 589, characters 6-24
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#get in file "source.ml", line 660, characters 10-41
2022/09/14 20:37:13 [clock.main:4] Called from VFrame.get_content in file "stream/vFrame.ml", line 45, characters 4-20
2022/09/14 20:37:13 [clock.main:4] Called from Video_effects.base#get_frame in file "operators/video_effects.ml", line 37, characters 12-41
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#instrumented_get_frame in file "source.ml", line 589, characters 6-24
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#get in file "source.ml", line 660, characters 10-41
2022/09/14 20:37:13 [clock.main:4] Called from Available.available#get_frame in file "operators/available.ml", line 40, characters 6-20
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#instrumented_get_frame in file "source.ml", line 589, characters 6-24
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#get in file "source.ml", line 660, characters 10-41
2022/09/14 20:37:13 [clock.main:4] Called from Add.add#get_frame.(fun) in file "operators/add.ml", line 119, characters 12-24
2022/09/14 20:37:13 [clock.main:4] Called from Stdlib__List.fold_left in file "list.ml", line 121, characters 24-34
2022/09/14 20:37:13 [clock.main:4] Called from Add.add#get_frame in file "operators/add.ml", line 108, characters 8-1023
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#instrumented_get_frame in file "source.ml", line 589, characters 6-24
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#get in file "source.ml", line 660, characters 10-41
2022/09/14 20:37:13 [clock.main:4] Called from Add.add#get_frame.(fun) in file "operators/add.ml", line 119, characters 12-24
2022/09/14 20:37:13 [clock.main:4] Called from Stdlib__List.fold_left in file "list.ml", line 121, characters 24-34
2022/09/14 20:37:13 [clock.main:4] Called from Add.add#get_frame in file "operators/add.ml", line 108, characters 8-1023
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#instrumented_get_frame in file "source.ml", line 589, characters 6-24
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#get in file "source.ml", line 660, characters 10-41
2022/09/14 20:37:13 [clock.main:4] Called from Add.add#get_frame.(fun) in file "operators/add.ml", line 119, characters 12-24
2022/09/14 20:37:13 [clock.main:4] Called from Stdlib__List.fold_left in file "list.ml", line 121, characters 24-34
2022/09/14 20:37:13 [clock.main:4] Called from Add.add#get_frame in file "operators/add.ml", line 108, characters 8-1023
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#instrumented_get_frame in file "source.ml", line 589, characters 6-24
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#get in file "source.ml", line 660, characters 10-41
2022/09/14 20:37:13 [clock.main:4] Called from Switch.switch#get_frame in file "operators/switch.ml", line 254, characters 14-39
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#instrumented_get_frame in file "source.ml", line 589, characters 6-24
2022/09/14 20:37:13 [clock.main:4] Called from Source.operator#get in file "source.ml", line 660, characters 10-41
2022/09/14 20:37:13 [clock.main:4] Called from Output.output#output in file "outputs/output.ml", line 166, characters 10-30

But in my opinion this is correct and this is type of issue shouldn't be solved increase text buffer but using different strategies such as on_end event handler.

Alternatively we could have an on_scroll event (where x is passed) to use to change things when a particular x value is reached.

[smimram]

I have just added an on_cycle option to register a handler when cycling. However it will only work when cycle=true is activated. When this is not the case, see my answer to #2622.