Hopefully will fix the crash in vkDestroyInstance on the NV Shield TV,
and likely reduce boot times slightly for drivers that take a while
to create instances.
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().
Should fix a possible reference to deleted framebuffers, as well as fixing
the issues with the render area being correct if the game's source area
changes, or auto-scaling is enabled.
