mesa: update the shader programs->TexturesUsed array at link time

mesa: update the shader programs->TexturesUsed array at link time

If an application never calls glUniform() to set sampler variable values
they'll remain 0 (the default value/unit).
Now call _mesa_update_shader_textures_used() at link time in case glUniform()
is never called.  program->TextureUsed[] will then be correct for state
validation.