effects: use RenderTarget directly

Instead of calling effects->renderTargetRect() and effects->renderTargetScale(), pass the actually used render target in all the necessary methods. This ensures that if an effect renders a screen with a modified scale or projection matrix, the modified values get used instead of the "global" ones

This MR has one caveat: In OffscreenData::maybeRender, effects->renderTargetScale() was used before. As the method is sometimes called completely outside of the compositing cycle, that isn't possible now anymore, so I assume a scale of 1, which isn't nice. While it wasn't exactly done right before either, it would be good to have a proper solution...