This refactor accomplishes a number of goals:

- make the pl_shader_av1_grain sample directly from `tex` instead of
  relying on pre-sampled tex, which allows it to correctly keep track of
  the texture scale even when sampling from the luma tex, and also gets
  rid of redundant fields
- makes the grain_state per-plane rather than shared, which avoids
  thrashing the grain state cache due to changing subsampling index
- fixes the channel order when applying grain to non-YCbCr content
- fixes some bugs related to grain scaling w.r.t texture sample depth

Tested to produce correct output on a 4:2:2 10-bit grain sample.