Wayland: Allow to take single screen screenshots using scale factor without loss

Summary:
The screenshot made on screens with scale factor were downscaled by their scale factor making them blurry.
It prevents taking screenshots of missing Hidpi related bugs showing the issues under Wayland.

This fix the case of a single screenshot, but not the rest:
Multiscreen screenshot downscales the screen using scale factor.
Spectacle rectangular selection screenshot is broken as soon as some scale factor different than 1 is used on any screen.

Test Plan:
Under Wayland with a scale factor on a screen, take a screenshot using spectacle.
The output image is not downscaled and has the same size as the screen resolution.

No other change to any other screenshot mode, or under X.

Reviewers: davidedmundson, KWin

Reviewed By: davidedmundson, KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D29010