Add a shortcut to pause/unpause display

[rom1v]

Pause on MOD+z, unpause on MOD+Shift+z.

It only impacts rendering, the device is still captured, the video stream transmitted to the device, and recorded if recording is enabled.

Fixes #1632

[rom1v]

@ttschnz (#1632 (comment)) please test the updated PR, keyboard and mouse should be disabled while in pause.

[rom1v]

Maybe Alt+z would be better (in reference to Ctrl+z which stops a process on Linux)? What do you think?

[ttschnz]

Here are two features I discovered to still work during the freeze:

1. volume can be changed via shortcut
2. files can be transferred (also apps)

I would argue for these two to stay, because:

1. We freeze the screen to pause updating what's happening on the screen, I see it as a feature to still be able to control volume.
2. For this action to be triggered, something bust be done on purpose, and as a end-user I wouldn't expect a screen-freeze to also disable this.

During my (short) tests, I came to one or two occurences where the frame would not update as soon as I unfreezed, allthough it would then continue as soon as there was another update (through direct input on the device or through an interaction with the stream). However, I could not reproduce it on purouse, I will reach out as soon as I know more about this - I will certainly use the feature extensively during the next week.

As of the shortcut, I've come to like mod+z, one could persume "z", as in freeze.

[rom1v]

volume can be changed via shortcut

Oops, I forgot this one. For consistency with other shortcuts actions, I think they should be disabled on pause.

Files can be transferred (also apps)

I think this one can be kept, it is quite independent.

During my (short) tests, I came to one or two occurences where the frame would not update as soon as I unfreezed

Oops, I did not update correctly. I started to fix, but there's still something missing. I will fix tomorrow.

[ttschnz]

Another flash of insight I just had: during the freeze you could add a shortcut that updates the screen, but then refreezes, as if I pressed mod+z twice. Since the inputs don't redirect to the device during the paused display, a simple r would be enough (allthough inconsistent with the default behaviour).
This could however be challenging to implement, and I also don't want to push this feature too far, KISS principle applies...

[rom1v]

Then I propose the following:

• MOD+z: pause (or refresh if already paused)
• MOD+Shift+z: unpause

Another benefit is that we don't need to know the current state (which is not always obvious) to pause or unpause. I updated the PR.

[ttschnz]

I've been testing commit b9cc6f7 during last and this week - I haven't found anything to complain about, so I'd say it's good like that.

For the shortcuts, I'd leave it the way it is, I find it convenient.

[rom1v]

Let's merge this 🚀

[Helaer]

@rom1v Hello, I found that although the screen is paused, there is still a large amount of CPU occupied on the computer side. Can you stop rendering processing on the computer side when the screen is paused?

[rom1v]

Can you stop rendering processing on the computer side when the screen is paused?

This feature just prevents the frame to be refreshed in the display. It does not impact other components.

The video stream is still transmitted, demuxed and decoded. The intermediate frames (at least a few ones) need to be decoded anyway, because the current frame (on unpause) may depend on them. In addition, other components may need the decoded frames (v4l2).

The feature is very local (non intrusive) to the display component, it does not impact other components. It's just a mean to "hide" a transition during a presentation, it does not release any resources.