[effects] Make screenshot effect work with multiple output rendering

Summary:
Adjusts the screenshot effect to mutliple output rendering. In that case
the blit from framebuffer can only get one output. Thus the effect needs
to blit every output and combine the rendering result.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D3059