dav1dplay: add --highquality to toggle render quality
dav1dplay: add --highquality to toggle render quality

Only meaningful with libplacebo. The defaults are higher quality than
SDL so it's an unfair comparison and definitely too much for slow iGPUs
at 4K res. Make the defaults fast/dumb processing only, and guard the
debanding/dithering/upscaling/etc. behind a new --highquality flag.

videolan/dav1d