wined3d: Add shader_get_param() fn, which processes address tokens.

wined3d: Add shader_get_param() fn, which processes address tokens.

Add a new function to process parameters.
On shaders 1.0, processing parameters amounts to *pToken++.
On shaders 2.0+, we have a relative addressing token to account for.
This function should be used, instead of relying on num_params everywhere.