Post Processing Does Not Account For Screen-Percentage Quantization

When changing r.ScreenPercentage the final value used for buffer generation is clamped to the nearest 4 pixels to aid in post processing and other steps. During post processing the same calculation for screen percentage is performed but this quantization does not happen creating up to a 3 pixel discrepancy. This leads to artifacts when buffer sizes and viewports mismatch when working with un-cleared data.

Both calculations should be identical to avoid edge cases which can be particularly apparent on consoles.

See the example in the UDN, or:
Launch QAGame in -game, set the resolution to 1600x900, set screen percentage to 50%, turn the camera and if correct there should be no black smear at the bottom of the screen.