WebGPU: Carry pcg generator state through stage

[TheBlek]

Before:

After:

From my understanding pseudo-random generators usually do not behave uniformly when sampling only a couple numbers before initializing with a new seed:

This has worse randomness:
init( 1 );
rand()
rand()
init( 2 );
rand()
rand()
...

Than this:
init( 1 );
rand()
rand()
rand()
rand()
...

Issues got more pronounced with more complex bsdfs in #717, but here it is also visible.

[gkjohnson]

Thanks - passing the bounces value into the seed was a fairly lazy fix I forgot to got back and check 😅

In terms of randomness getting the "sobol" random function working is probably our better bet for stratified sampling functions. It provides a better result and should still be applicable for the wavefront path tracing approach. See this tweet