Summary:
The glCopyTex(Sub)Image calls are adjusted to take the per output
rendering into account. In addition contrast effect sets the model view
projection matrix in each call to ensure it's on the correct screen.

Blur probably needs more changes for the cached texture to work, but
it's a start.

Test Plan: Blur and Background contrast work on multi-screen wayland

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3101