mesa/swrast: fix inverted front buffer rendering with old-school swrast

I've no idea when this broke, but we have some people who wanted it fixed,
so here's my attempt.

reproducer, run readpix with swrast hit f, or run trival tri -sb things are
upside down, after this patch they aren't.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=62142
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66213

Cc: <mesa-stable@lists.freedesktop.org>"
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/mesa/drivers/dri/swrast/swrast.c

@@ -406,8 +406,8 @@ swrast_map_renderbuffer(struct gl_context *ctx,
 				     (char *) xrb->Base.Buffer,
 				     dPriv->loaderPrivate);
 
-      *out_map = xrb->Base.Buffer;
-      *out_stride = stride;
+      *out_map = xrb->Base.Buffer + (h - 1) * stride;
+      *out_stride = -stride;
       return;
    }