dynamic offsets need to be divisible by 4 because of shader memory model atomicity guarantees

```webidl
void setBindGroup(u32 index, GPUBindGroup bindGroup, optional sequence<u64> dynamicOffsets);
```

Those dynamicOffsets need to be divisible by 4 because of the memory model.

I'm just documenting this requirement here until we have a real spec and can start documenting this stuff properly.