Make GeForce Now work on DXVK

[Joshua-Ashton]

• Fix a refcounting issue with additional swapchains.
• Fix a race condition that would cause video conversion formats to be black when used with D3DLOCK_DISCARD
  • This is really just a band aid.
  • Maybe this could be fixed better, the problem is that the conversion uses it's own context and when the buffer was invalidated, the conversion's context has no way of knowing that, so it was reading from the old slice still.
  • The correct solution would be to maybe move the conversion onto the main CS thread, that's mildly annoying but probably not impossible.
• Cleanup YUV helpers
• Implement NV12
  • And a test with a cute lil froggy image!