st/nine: Fix ff calculation of midVec

In case of non local viewer the value has to be subtracted.
Fixes failing WINE tests in test_specular_lighting() (visual.c)

Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Axel Davy <axel.davy@ens.fr>

src/gallium/state_trackers/nine/nine_ff.c

@@ -800,9 +800,9 @@ nine_ff_build_vs(struct NineDevice9 *device, struct vs_build_ctx *vs)
             /* midVec = normalize(hitDir + eyeDir) */
             if (key->localviewer) {
                 ureg_normalize3(ureg, rMid, ureg_src(rVtx), tmp);
-                ureg_ADD(ureg, rMid, ureg_src(rHit), ureg_negate(ureg_src(rMid)));
+                ureg_SUB(ureg, rMid, ureg_src(rHit), ureg_src(rMid));
             } else {
-                ureg_ADD(ureg, rMid, ureg_src(rHit), ureg_imm3f(ureg, 0.0f, 0.0f, 1.0f));
+                ureg_SUB(ureg, rMid, ureg_src(rHit), ureg_imm3f(ureg, 0.0f, 0.0f, 1.0f));
             }
             ureg_normalize3(ureg, rMid, ureg_src(rMid), tmp);
             ureg_DP3(ureg, ureg_saturate(tmp_y), ureg_src(rNrm), ureg_src(rMid));