Add mode option to mirror

[TRPB]

Adding the config option is step 1, I'll open another PR in the Wayfire branch which checks the option and sets the mode.

This is step 1, I'll also add a the a PR to the Wayfire branch which checks this.

This adds support for a config option like:

[option:DP-2]
mode = mirror DP-1 1920x1080@60000

Reason

Currently Mirroring works like this:

          case output_config::MODE_MIRROR:
            state.source = OUTPUT_IMAGE_SOURCE_MIRROR;
            state.mode   = select_default_mode();
            state.mirror_from = mode.get_mirror_from();
            break;

This sets the monitors default mode and mirrors another output. This can have side effects and unexpected behaviour when the mirrors resolution and refresh rate don't match the outputs. For example, if you mirror a 60hz screen on a 144hz screen the frames don't sync up neatly and it looks strangely jerky, by forcing the output to 120hz or 60hz there are fewer display issues.

There are also times when we might not want to use the default mode. I might prefer my monitor to run at the same resolution of the display being mirrored to scale in the hardware rather than software.

While we could just copy the source's mode here, we might try to copy a monitor that runs at 60hz to a monitor that runs at 59.9hz, not solving the problem. Rather than trying to guess at a best match, let's let the user decide.

My next PR is going to adjust the logic above to set the mode from the config if it's set, otherwise use the existing behaviour.

[ammen99]

The feature makes sense in general. However I think the implementation should be made to just use sscanf multiple times.

[ammen99]

I'd like to also add some checks in the test that this is properly parsed.

[ammen99]

Formatting: { on a new line.

[ammen99]

I am confused, is the code not supposed to continue running after that, so that we can parse the mode as well?

Also, I think it may be simpler (and more flexible wrt. whitespace handling) to try sscanf multiple times. One time with a format string like mirror %s <optionally the mode>, and if that fails, then scan a normal resolution, like the code below does.

[mark-herbert42]

59-1.txt
I've modified the logic a bit so the program execution is not prematurely finished before the mode string parsed. So now it works - but the code is a bit ugly. I am not C++ expert ufortunately. But for personal use that works.