wined3d: Add code for using register combiners for texture stage operations.

wined3d: Add code for using register combiners for texture stage operations.

On nVidia cards the value of GL_MAX_TEXTURE_UNITS is generally not
larger than 4. In Direct3D that would correspond to
MaxSimultaneousTextures in the caps, rather than MaxTextureBlendStages
(which can be much larger) to which it currently corresponds in
wined3d. Using register combiners we can get around that limitation
and get up to GL_MAX_GENERAL_COMBINERS_NV (typically 8) texture
stages. This patch adds code for doing the texture operations with
register combiners instead of ARB_texture_env_combine or
NV_texture_env_combine4, but doesn't make use of that code yet. That's
what the next patch will do.