st/nine: Fix two special cases in ff ps

if first alpha stage is disabled and writes to temp,
diffuse alpha is written to temp.
Last stage always writes to current.

Behaviour was deduced by tests with a test app.

Signed-off-by: Axel Davy <axel.davy@ens.fr>

src/gallium/state_trackers/nine/nine_ff.c

@@ -1773,6 +1773,26 @@ nine_ff_get_ps(struct NineDevice9 *device)
         }
     }
 
+    /* Note: If colorop is D3DTOP_DISABLE for the first stage
+     * (which implies alphaop is too), nothing particular happens,
+     * that is, current is equal to diffuse (which is the case anyway,
+     * because it is how it is initialized).
+     * Special case seems if alphaop is D3DTOP_DISABLE and not colorop,
+     * because then if the resultarg is TEMP, then diffuse alpha is written
+     * to it. */
+    if (key.ts[0].colorop != D3DTOP_DISABLE &&
+        key.ts[0].alphaop == D3DTOP_DISABLE &&
+        key.ts[0].resultarg != 0) {
+        key.ts[0].alphaop = D3DTOP_SELECTARG1;
+        key.ts[0].alphaarg1 = D3DTA_DIFFUSE;
+    }
+    /* When no alpha stage writes to current, diffuse alpha is taken.
+     * Since we initialize current to diffuse, we have the behaviour. */
+
+    /* Last stage always writes to Current */
+    if (s >= 1)
+        key.ts[s-1].resultarg = 0;
+
     key.projected = nine_ff_get_projected_key(state);
     key.specular = !!state->rs[D3DRS_SPECULARENABLE];