mesa: move sampler state into new gl_sampler_object type

gl_texture_object contains an instance of this type for the regular
texture object sampling state.  glGenSamplers() generates new instances
of gl_sampler_object which can override that state with glBindSampler().

src/mesa/program/prog_statevars.c

@@ -582,7 +582,7 @@ _mesa_fetch_state(struct gl_context *ctx, const gl_state_index state[],
                value[0] =
                value[1] =
                value[2] =
-               value[3] = texObj->CompareFailValue;
+               value[3] = texObj->Sampler.CompareFailValue;
             }
          }
          return;