Should we ever introduce anything else that has to be done when a command
buffer is executed (e.g. invalidating constants from previous commit), we
don't have to update all the callers.
There's not a lot of point in passing these around or storing them
(texture cache/state tracker mainly) as there will only ever be a single
instance of the class.
Also adds downcast helpers such as Vulkan::Renderer::GetInstance().
Stenzek