The "VK" prefix predates the "Vulkan" namespace. It was carried around
the codebase for consistency. "VKDevice" currently is a bad alias with
"VkDevice" (only an upcase character of difference) that can cause
confusion. Rename all instances of it.
Migrates the video core code closer to enabling variable shadowing
warnings as errors.
This primarily sorts out shadowing occurrences within the Vulkan code.
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.
