Cycles: Separate CPU & GPU tile size controls

The new Hybrid option for Cycles rendering is a fantastic idea but is let down by the fact that GPU favors larger render tiles, and CPU favors small render tiles.

In my tests, (GTX 1070, GTX 970, and i7-6700k) rendering pure GPU with optimal tile size is always faster than Hybrid with any tile size I've tested. This is because too small of tiles means the GPU runs slower than it should, and too big of tiles means the render usually gets stuck waiting for the last CPU tiles to finish.

I understand this won't always be the case, but what I propose is the option to change the tile sizes independently so you can set the optimum tile size for both.