wined3d: Enable clip planes with GLSL shader backend

[Ivan Gyurdiev]

> +    if (use_vs(stateblock->wineD3DDevice) && stateblock->wineD3DDevice->vs_selected_mode == SHADER_ARB) 
GLSL specific logic should go in glsl_shader.c.

Please try to get away from writing code in terms of flags and if 
statements - use the OOP model.
Backend-specific logic should be removed from pixelshader and 
vertexshader too...