fix span->facing computation and gl_FrontFacing initialization

src/mesa/swrast/s_fragprog.c

@@ -120,7 +120,7 @@ init_machine(GLcontext *ctx, struct gl_program_machine *machine,
    /* if running a GLSL program (not ARB_fragment_program) */
    if (ctx->Shader.CurrentProgram) {
       /* Store front/back facing value in register FOGC.Y */
-      machine->Attribs[FRAG_ATTRIB_FOGC][col][1] = (GLfloat) span->facing;
+      machine->Attribs[FRAG_ATTRIB_FOGC][col][1] = 1.0 - span->facing;
    }
 
    machine->CurElement = col;