d3d12_copy_manager

[vk2gpu]

For initial data uploads we need something better than what is currently implemented. What would be desired:

• Implemented outside of the d3d12_device.
• Independent of what frame we're on.
• Thread safe.
• As foolproof synchronization as possible (i.e. fence will automatically be added when a resource is used that has a pending upload)
• Use copy queues.
• Host visible upload memory preferably will not resize. Either upload piece meal, or one shot allocation for large resources.