shaders/film_grain: move H.274 model values out of struct

This reduces the sizeof(struct pl_film_grain_data), and anything that depends on it (e.g. sizeof(struct pl_frame) or sizeof(struct pass_state)) by around 10 kB, bringing pl_frame down to only 512 bytes.

The massive size of these structs was never intended. This change only affects the AVFrame utilities, which can simply reference the existing side data because the pl_frames generated from them were already specified to depend on the lifetime of the corresponding AVFrame.