obs-frontend-api: Add event for video reset

[walker-WSH]

Description

If obs_reset_video is called in OBS, for 3rd plugins, they should re-create the obs_output or video_output hold in their code. Otherwise, 3rd plugins will easily crash since their video_output and obs_output::video_t have been deleted in obs_reset_video.

Motivation and Context

if a 3rd plugin hold their obs_output or video_output, crash will easily happen after below operation:

install ndi plugin
open settings of OBS, to video page
modify resolution
click OK
Tool menu of OBS, select NDI, start its output, crash
the reason can be found in DistroAV/DistroAV#1096

Except NDI plugin, I also found this kind of crash on other plugin, such as vertical-canvas, source-record.
If OBS provide this new front event, 3rd plugin has the chance to reset their obs_output or video_output.

How Has This Been Tested?

settings -> video -> modify resolution -> click OK -> check if this event can be sent

Types of changes

New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[tt2468]

This should be implemented as a core OBS signal, not as a frontend event. Resets can happen outside of the frontend, in which case this event would not be fired.

[walker-WSH]

hi @tt2468
I commit another PR to add new signals in libobs, please review it, thanks~
#11686

so I close this PR