Steps to reproduce:
1. Open user attached project "CompositeImages.zip" in Unity 5.6
2. Select RainCone material in project window and set both opacity colours to white (It will make it easier to see the effect)
3. Open "compositeCameras" scene and enter play mode
4. Observe how both cameras that take up 50% of the screen also have 50% of the effect on them effectively having only 25% of screen space each

Expected result: Instead of cutting off the effect at 50% the effect camera should use 50% of the effect and stretch it over the entire camera viewport. Making it so that each camera is rendering 50% of the screen and has 100% of its screen space covered by the effect.
Actual result: Having a camera render effects with a viewport of width 0.5 onto a camera with a viewport of width 0.5 multiplies and ends up with 0.25 of the screen space covered with the effect instead of the 0.5

Reproduced on 5.6.0f3, 5.6.1p1, 2017.1.0b6
Regression introduced in 5.6.0a1
Does not reproduce on: 5.4.5.p1, 5.5.3p3

Notes: To observe effect on 5.4.5 and 5.5.3 open user attached project in https://oc.unity3d.com/index.php/s/tQgWTHoLrKJY7dd/download?path=%2F&files=CompositeImages5.4.4p4.rar