Reduces some header churn and reduces rebuilds when some header
internals change.
While we're at it we can also resolve a missing include in buffer_cache.
This abstractio represents the state of the 3D engine at a given draw.
Instead of changing individual bits of the pipeline how it's done in
APIs like D3D11, OpenGL and NVN; on Vulkan we are forced to put
everything together into a single, immutable object.
It takes advantage of the few dynamic states Vulkan offers.
