Move resource creation, readbacks, and InvocationState from DX backend to shared file. (#1330)

Move code that doesn't need to live in the DX backend to a shared file.

Why make this change?
1. All this code should eventually live the offloader application.
However, it can't live there right now because the rendering backends
are too divergent.
2. In a follow-up PR we can reuse this code in the Vulkan and Metal
backends resulting in a very significant reduction in code size.

This PR doesn't make any other code changes other than splitting the
creation of the descriptor tables into a separate function from resource
creation, due to no abstraction existing for building descriptor
sets/tables and binding those yet.

Author: manon-traverse