There's a lot to do for RWTexture and RWBuffer, so it will be broken up into
several PRs.  This is #1.

This adds RWTexture and RWBuffer support, with the following limitations:
  * Only 4 component formats supported
  * No operator[] yet

Those will be added in other PRs.

This PR supports declarations and the Load & GetDimensions methods.  New tests are
added.