This is can be used to apply AV1 film grain onto planes directly on the
GPU. Eventually, the pl_renderer will be able to take this grain params
struct and do it automatically - but for now, we need the basic shader
implemntation first.

We make some amount of effort to follow the AV1 spec exactly, even down
to reproducing the (terrible) PRNG, awkward corner blending and rounding
(where possible).

Some things are not currently preserved: In particular, the blending of
pixels is not correctly rounded. (We continue calculating on the
un-rounded floating point values) In practice this does not produce any
significant deviation (bounded by +/- 1) - especially if you don't round
but instead continue calculating with floating point precision, in which
case the true error is only the difference between the un-rounded value
and its correctly rounded counterpart (bounded by +/- 0.5).

The generated shader can be used as both a compute shader and a fragment
shader, but a compute shader is preferred (slightly faster).

We upload the weights and grain tables using an SSBO (which we need
anyway to store the integer offset table). In theory we could try
replacing these by LUTs, but the existing sh_lut mechanism does not lend
itself well to frequent updates, and we will want to be updating this
texture constantly (especially for parallel streaming).

But more testing is warranted here. For now, this should be a working v1
implementation.