mesa: fix DIFFERENT_SIGNS() function

Looks like converting this to a macro, returning bool, caused us to
lose the high (31st) bit result.  Fixes piglit fbo-1d test.  Strange
that none of the other tests I ran caught this.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=54365

Tested-by: Vinson Lee <vlee@freedesktop.org>

src/mesa/main/macros.h

@@ -711,7 +711,7 @@ DIFFERENT_SIGNS(GLfloat x, GLfloat y)
    fi_type xfi, yfi;
    xfi.f = x;
    yfi.f = y;
-   return (xfi.i ^ yfi.i) & (1u << 31);
+   return !!((xfi.i ^ yfi.i) & (1u << 31));
 #else
    /* Could just use (x*y<0) except for the flatshading requirements.
     * Maybe there's a better way?