[vp8] Use unfiltered source frame for PSNR with denoiser

When denoising is enabled (noise_sensitivity > 0), vp8_denoiser_filter_uv()
and vp8_de_noise() modify cpi->Source in-place during encoding. Previously,
generate_psnr_packet() and internal PSNR stats compared the reconstructed
frame against cpi->Source after it had been modified in-place by the
denoiser.

Preserve a copy of the unfiltered source frame in cpi->raw_source_frame
when PSNR calculation and denoising are enabled, and use it as the
reference frame for PSNR calculation.

Change-Id: If125ac32961c4007ce18944657eda57618a97df0
Bug: 571976041