Tile stealing may steal a CPU tile buffer and move it to the GPU, but next time around that
tile may be re-used on the CPU again (in progressive refinement mode). The buffer would
still be on the GPU then though, so is inaccessible to the CPU. As a result Blender crashed
when the CPU tried to write results to that tile buffer.
This fixes that by ensuring a stolen tile buffer is moved back to the device it is used on before
rendering.
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