Postprocessing does not handle camera's viewport correctly

[mvaligursky]

repro: https://playcanvas.com/editor/scene/1103574
forum thread: https://forum.playcanvas.com/t/trouble-with-double-camera/18460/12
related fix: #2939

if post processing is applied on camera with smaller viewport (minimap), the postprocessing gets clipped to the viewport which it should not .. it should apply to whole render target