vulkan: expose pl_vulkan_choose_device

This is a useful helper function for scripts. I debated whether or not to just copy/paste the options struct or if we should retroactively change the existing one for aesthetics, but I'm tired of breaking the API for an insufficiently good reason. Plus, we need to set a different instance anyway, so this isn't too bad.

Another alternative would have been to re-use the actual pl_vulkan_params but this might be even more confusing. Shrug

Closes #74 (closed).