Move global random generator into kernel private random generator

[keryell]

While https://github.com/triSYCL/path_tracer/pull/21 added a faster random generator for accelerators, it cannot yet work on a real device because it is a global variable not accessible from a SYCL kernel on a plain device. More generally, any global variable should be removed.

[keryell]

Now we have the executor for either parallel_for or single_task we can declare the random generator there to have 1 instance per work-group. This is trivial for single_task (1 work-item, so 1 work-group :-) ). For parallel_for, either

• use hierarchical parallelism with the random generator declared as local (in the sense of C++) variable in the work-group scope or the
• use the same parallel_for as now but then pass a local_accessor of 1 random generator structure (containing the random generator and the std::uniform_real_distribution<float>) and think about calling explicitly the constructor, since local_accessor provide raw uninitialized memory.

[keryell]

Thank you for the experiment in https://github.com/triSYCL/path_tracer/pull/37 Very useful! This global rng state is a fundamental nightmare and I was wrong thinking that just having one local rng to the work-group is enough to solve the inter-work-group interference. This is then the same problem at the work-item level inside each work-group... :-( We could extend SYCL to introduce the concept of thread local id... :-) But this is painful and it would probably necessary to pass it as a parameter anyway. So my id about hierarchical parallelism to have a local rng to the work group does not work so there is no need to use it, Just a parallel_for is enough, For FPGA it would be interesting to see how HLS can pipeline the code, probably having the rng executed in dataflow?

[keryell]

Implemented with https://github.com/triSYCL/path_tracer/pull/37