wgl: Fix visual's buffer_mask configuration.

diff --git a/src/gallium/state_trackers/wgl/stw_pixelformat.c b/src/gallium/state_trackers/wgl/stw_pixelformat.c
index 18ac482..5ede1d4 100644 (file)
--- a/src/gallium/state_trackers/wgl/stw_pixelformat.c
+++ b/src/gallium/state_trackers/wgl/stw_pixelformat.c
@@ -185,7 +185,7 @@ stw_pixelformat_add(
     */
    pfi->stvis.buffer_mask = ST_ATTACHMENT_FRONT_LEFT_MASK;
    if (doublebuffer)
-      pfi->stvis.buffer_mask = ST_ATTACHMENT_BACK_LEFT_MASK;
+      pfi->stvis.buffer_mask |= ST_ATTACHMENT_BACK_LEFT_MASK;
 
    pfi->stvis.color_format = color->format;
    pfi->stvis.depth_stencil_format = depth->format;