Summary:
Add a shouldReturnNativeSize to screenshotFullscreen so that KWin can export
screenshot that are screen pixel accurate.

Useful for spectacle to be able to do rectangular selection kind of screenshot.

Test Plan:
Example of a top screen using a 1.25 scale ratio being export in native resolution
{F8255144}
(The top screen has a bigger size than its virtual geometry and next screen doesn't overlap)

Example of the same screen in virtual resolution:
{F8255146}

Reviewers: #kwin, davidedmundson, bport, zzag, apol

Reviewed By: #kwin, davidedmundson

Subscribers: broulik, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29122