Offsetting kernel indices

[glwagner]

It'd be extremely useful to have a feature for offsetting the kernel index, either at launch time or setup time.

Not sure what the best design or interface would be. @simone-silvestri and I discussed possibly embellishing StaticSize:

https://github.com/JuliaGPU/KernelAbstractions.jl/blob/4128c3db26f4b9fef59d9589655ea96c35ea2db1/src/nditeration.jl#L14-L17

Our work exclusively uses statically sized kernels so I think we'd prefer a way to specify the offsets statically; eg the size and offset are simultaneously determined.

But another possibility would be to accept offsets as a kwarg similar to how ndrange is accepted for DynamicSize?

[vchuravy]

I talked with Simone and I think expressing this as an Index projection similar to LinearIndicies and CartesianIndices is the right direction.

[b-fg]

In WaterLily we could also benefit from this as we are currently passing index offsets manually too.