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>

diff --git a/src/mesa/drivers/dri/swrast/swrast.c b/src/mesa/drivers/dri/swrast/swrast.c
index c062071b09b53f5be104e4fb64c1a7e09989c124..73dc5c4c06161974cee3584d7b7b1b8240df6012 100644 (file)
--- a/src/mesa/drivers/dri/swrast/swrast.c
+++ b/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;
    }