Blender Git Loki

Cycles X: Run init_from_camera kernel for all tiles

Avoids pointer magic which is not necessarily supported by all compute
backends and allows to ensure there are no extra latency caused by
multiple kernel launches.

Currently this does not bring performance improvements, but this change
opens doors for more compute backends and makes it possible to test
different tile slicing and scheduling strategies.

```
init_all_tiles cycles-x
bmw27.blend 10.3444 10.326
classroom.blend 16.476 16.6067
pabellon.blend 9.13914 9.13556
monster.blend 11.9673 11.963
barbershop_interior.blend 12.4566 12.4414
junkshop.blend 16.4764 16.491
pvt_flat.blend 17.288 17.2757
```

Differential Revision: https://developer.blender.org/D11304