FSR EASU and FSR2 sharpen fragment shaders had a manual Y-flip to undo
the now-removed postprocess.vert flip. Strip those since the vertex
shader no longer flips, making all postprocess paths consistent.
Also flip the default mouse Y-axis to match user expectation (mouse
down = look up / flight-sim style) and make FSR1 disable MSAA on
enable, matching FSR2 behaviour (FSR provides its own spatial AA).