Move code around in the compositor

Doing a bit of refactoring on the compositor and would like some comments. No functional change has happened.

• Use a bit more common code in comp_vk_swapchain.c
• Move truly common Vulkan code to auxiliary
• Improve some comments
• Move comp_vk_swapchain.c into main.
• Make the state tracker create the main FD compositor and pass it in as a argument to the client code.
• In the build also spilt main & client code.