frontend/nine: Fix ff position_t fallback when w = 0

For post-transformed vertices, w = 0 is similar to
w = 1. Replace the value to fix rcp(w).

It is common for apps to pass w = 0 for
position_t.

cc: mesa-stable

Signed-off-by: Axel Davy <davyaxel0@gmail.com>
Acked-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <mesa/mesa!18021>
(cherry picked from commit b5df2056)

.pick_status.json

@@ -904,7 +904,7 @@
         "description": "frontend/nine: Fix ff position_t fallback when w = 0",
         "nominated": true,
         "nomination_type": 0,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null
     },

src/gallium/frontends/nine/nine_ff.c

@@ -455,6 +455,9 @@ nine_ff_build_vs(struct NineDevice9 *device, struct vs_build_ctx *vs)
             ureg_ADD(ureg, ureg_writemask(tmp, TGSI_WRITEMASK_XY), ureg_src(tmp), ureg_imm1f(ureg, -1.0f));
             /* Y needs to be reversed */
             ureg_MOV(ureg, ureg_writemask(tmp, TGSI_WRITEMASK_Y), ureg_negate(ureg_src(tmp)));
+            /* Replace w by 1 if it equals to 0 */
+            ureg_CMP(ureg, ureg_writemask(tmp, TGSI_WRITEMASK_W), ureg_negate(ureg_abs(ureg_scalar(ureg_src(tmp), TGSI_SWIZZLE_W))),
+                     ureg_scalar(ureg_src(tmp), TGSI_SWIZZLE_W), ureg_imm1f(ureg, 1.0f));
             /* inverse rhw */
             ureg_RCP(ureg, ureg_writemask(tmp, TGSI_WRITEMASK_W), _W(tmp));
             /* multiply X, Y, Z by w */