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