Use CPU and GPU for transcoding (parallel / fallback)

If I enable HW transcoding it uses the iGPU or dedicated GPU (whatever is primary) and never the CPU. Instead I like to have a setting where I’m able to:

1.) Select the primary transcoding GPU
2.) Select fallback GPUs/CPUs and sort them in a list

Something like this:
Sort transcoding processor priority

Early 2021 clean-up: duplicate