d3dx9: Implement D3DXDeclaratorFromFvF

[James Mckenzie]

>Hi,
>
>I think I solved problems with this patch:
>
>Use the already implemented D3DXGetDeclVertexSize to compute the offset.
>This saves to introduce new internal functions.
>Simplify the checks for the index.
>Clean a little bit the code
>
David:

It is customary to label retries of patch submissions as Try 2, Try 3, and so on in increasing numbers somewhere in the patch name so that Alexandre will look at your latest attempt.

Thank you.

James McKenzie