More options in the "Performance > Device" drop down (2.8)

A simple feature.

With the inclusion of CPU/GPU co-rendering there are times when it would be faster or preferable to do a GPU only render. Such as when rendering a small portion of a scene at a high sample rate, the GPU will often finish all its tiles while the CPUs are still working on the ones they started on.

Currently, in order to achieve GPU only rendering I have to use the preferences and turn off the CPU for CUDA. This may be easier to solve by adding an additional option in the drop down.

I propose adding an option to the drop down for "Device" under the "Performance" menu, and (maybe?) renaming to make their implications more clear:

Performance>Device:
GPU+CPU
GPU Only
CPU Only