[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