i965: Fix fog coordinate g,b,a values when glFrontFacing isn't used.

Previously, we would sample (f,glFrontFacing,undef,undef) instead of the
(f,0,0,1) that fragment.fogcoord is supposed to return.  Due to
glFrontFacing's presence in FOGC.y, we'll still give bad results there when
glFrontFacing is used.

Bug #19122, piglit testcase fp-fog.

src/mesa/drivers/dri/i965/brw_wm_fp.c

@@ -363,6 +363,56 @@ static void emit_interp( struct brw_wm_compile *c,
 		 src_undef());
       }
       break;
+   case FRAG_ATTRIB_FOGC:
+      /* The FOGC input is really special.  When a program uses glFogFragCoord,
+       * the results returned are supposed to be (f,0,0,1).  But for Mesa GLSL,
+       * the glFrontFacing and glPointCoord values are also stashed in FOGC.
+       * So, write the interpolated fog value to X, then either 0, 1, or the
+       * stashed values to Y, Z, W.  Note that this means that
+       * glFogFragCoord.yzw can be wrong in those cases!
+       */
+
+      /* Interpolate the fog coordinate */
+      emit_op(c,
+	      WM_PINTERP,
+	      dst_mask(dst, WRITEMASK_X),
+	      0,
+	      interp,
+	      deltas,
+	      get_pixel_w(c));
+
+      /* Move the front facing value into FOGC.y if it's needed. */
+      if (c->fp->program.UsesFrontFacing) {
+	 emit_op(c,
+		 WM_PINTERP,
+		 dst_mask(dst, WRITEMASK_Y),
+		 0,
+		 interp,
+		 deltas,
+		 get_pixel_w(c));
+      } else {
+	 emit_op(c,
+		 OPCODE_MOV,
+		 dst_mask(dst, WRITEMASK_Y),
+		 0,
+		 src_swizzle1(interp, SWIZZLE_ZERO),
+		 src_undef(),
+		 src_undef());
+      }
+
+      /* Should do the PointCoord thing here. */
+      emit_op(c,
+	      OPCODE_MOV,
+	      dst_mask(dst, WRITEMASK_ZW),
+	      0,
+	      src_swizzle(interp,
+			  SWIZZLE_ZERO,
+			  SWIZZLE_ZERO,
+			  SWIZZLE_ZERO,
+			  SWIZZLE_ONE),
+	      src_undef(),
+	      src_undef());
+      break;
    default:
       emit_op(c,
 	      WM_PINTERP,