Shutdown QVK in R_Shutdown() by calling R_ShutdownContext().

The QVK shutdown is necessary, otherwise we'll leak all(?) allocated
Vulkan resources in case of a renderer restart by `vid_restart` and the
like. The also fixes some crashes in RADV at renderer shutdown.

src/client/refresh/vk/vk_rmain.c

@@ -1348,6 +1348,7 @@ void R_Shutdown (void)
 
 	Mod_FreeAll();
 	Vk_ShutdownImages();
+	R_ShutdownContext();
 }
 
 /*