There are several spaces that have to be considered in `GLRenderTarget::blitFromFramebuffer`:
* KWin logical space: the origin is located at the global top-left corner
* display space: the origin is located at the top-left corner of monitor/display
* OpenGL screen space: the origin is located at the bottom-left corner of monitor/display
Given `s`, which is in the KWin logical space, we have to transform it to the display space, then to the OpenGL screen space:
* KWin logical space -> display space: `y' = s.y() - s_virtualScreenGeometry.y()`
* display space -> OpenGL screen space: `y'' = s_virtualScreenGeometry.height() - y'`
Overall, `srcY0` and `srcY1` should be written as following
```
srcY0 = s_virtualScreenGeometry.height() - (s.y() - s_virtualScreenGeometry.y() + s.height())
srcY1 = s_virtualScreenGeometry.height() - (s.y() - s_virtualScreenGeometry.y())
```