pass through sample index to pixel history
Add a couple of utility functions for DXGI_FORMAT
Add MSAA support for pixel history. Refs #82

* We expand the depth-pixel-copy CS to copy colour pixels too, and be able
  to read from MSAA textures where appropriate.
* This introduces a small unnecessary overhead for non-MSAA colour textures
  but it's not really an issue.
* I've also changed it to use the existing textures rather than creating
  copies when the existing texture has BIND_SRV already.