Store GLRenderTarget::scale
Summary:
GLRenderTarget already stores the translation from global compositor
space to the render target co-ordinates, we also need a scale factor for
the exact same reasons.
Test Plan: Used in various effects after this patch and it works there.
Reviewers: Plasma
Subscribers: plasma-devel, kwin, KWin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D4948