main: Add Vulkan support
For machines with dual-GPUs by 2 different vendors, setting the VK_ICD_FILENAMES envvar reduces the GPUs it can detect to those of that vendor, effectively forcing a particular GPU to be chosen.
For machines with dual-GPUs, the variable will have no effect, and things will work as if no selection had been made, just like for NVidia GPUs.
Closes: #23 (closed)