dav1dplay: support on-GPU film grain synthesis

Annoying minor differences in this struct layout mean we can't just
memcpy the entire thing. Oh well.

Note: technically, PL_API_VER 33 added this API, but PL_API_VER 63 is
the minimum version of libplacebo that doesn't have glaring bugs when
generating chroma grain, so we require that as a minimum instead.

(I tested this version on some 4:2:2 and 4:2:0, 8-bit and 10-bit grain
samples I had lying around and made sure the output was identical up to
differences in rounding / dithering.)
33 jobs for dav1dplay_grain in 11 minutes and 59 seconds (queued for 4 minutes and 25 seconds)
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