RENDERER: improved vulkan renderer - dynamic renderer selection in the ui

[mgerhardy]

see issue #400

some older commits that I think might be useful here. It would be great if several people could test this before this is merged.

[mgerhardy]

Need to link moltenvk on mac

...VKimp_SetMode()...
 bpp 32    SDL_PIXELFORMAT_ARGB8888    1920 x 1080, refresh_rate: 144Hz
 MODE: 3, 640 x 480, refresh rate: 60hz
^3 Couldn't create a window: Failed to load Vulkan Portability library
----- Client Shutdown (Client fatal crashed: VKimp_Init() - could not load Vulkan subsystem: Failed to load Vulkan Portability library) -----
Hunk_Clear: reset the hunk ok

but need to check why - SDL_Vulkan_LoadLibrary should do the dynamic loading for us