Add QPRD support as subme=10
Refactor trellis lambda selection to be done in analyse_init instead of in trellis. This will allow for more easy adaption of lambda later on; for now it allows constant lambda across variable QPs. QPRD is only available with adaptive quantization enabled and generally improves SSIM and visual quality. Additionally, weight the SSD values from RD based on the relative QP offset for chroma; helps visually at high QPs where chroma has a lower QP than luma. This fixes some visual artifacts created by QPRD at high QPs. Note that this generally hurts PSNR and SSIM, and so is only on when psy-RD is on.
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