[Cin] CinGG crash rendering high fps

[Andrea paz]

I think this is important information to put in the manual. I would
like to look into it first to understand how CinGG works. As a further
cue we can see Cinelerra-CVE by Einar Rünkaru. He implemented the PTS
to be able to manage in timeline sources with different framerates,
resolutions and sizes. In the readme of his Github is written
(https://github.com/vanakala/cinelerra-cve):

"- Another huge change implementation of pts based timing. This enables
to edit media with variable framerate and get rid of assumption
that audio and video start simultaneously."
"- implemented pts based timing on video and audio making possible
    projects without constant framerate"

To do this he had to rewrite much of the program losing compatibility
with other versions of Cinelerra. So I don't think it's possible to
implement it in CinGG; but it may be useful to do a comparison to see
how it works in CinGG.

Here instead is information on how Lumiera (a total rewrite of
Cinelerra) is designed:

https://lumiera.org/documentation/user/intro/intro.html

Also interesting is the part about Cinelerra's (insurmountable)
limitations that led them to do a separate project:

https://www.lumiera.org/project/background/history/CinelerraWoes.html