Scaling support in ColorPicker effect

Summary:
glReadPixels needs the correct location relative to the
framebuffer so we need to factor in the scale as well as translation
when going from compositor space.

Test Plan:
Ran the plasma colour picker plasmoid in windowed mode
Clicked on multiple parts of a window
It was right every time

Reviewers: #plasma

Subscribers: plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D4950