x11: take ownership rather than adding reference for new renderbuffers

This avoids locking in the reference calls and fixes a leak after the
RefCount initialisation was change from 0 to 1.

Fixes: 32141e53d1 (mesa: tidy up renderbuffer RefCount initialisation)

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/mesa/drivers/x11/xm_api.c

@@ -311,8 +311,8 @@ create_xmesa_buffer(XMesaDrawable d, BufferType type,
    b->frontxrb->Parent = b;
    b->frontxrb->drawable = d;
    b->frontxrb->pixmap = (XMesaPixmap) d;
-   _mesa_add_renderbuffer(&b->mesa_buffer, BUFFER_FRONT_LEFT,
-                          &b->frontxrb->Base.Base);
+   _mesa_add_renderbuffer_without_ref(&b->mesa_buffer, BUFFER_FRONT_LEFT,
+                                      &b->frontxrb->Base.Base);
 
    /*
     * Back renderbuffer
@@ -328,8 +328,8 @@ create_xmesa_buffer(XMesaDrawable d, BufferType type,
       /* determine back buffer implementation */
       b->db_mode = vis->ximage_flag ? BACK_XIMAGE : BACK_PIXMAP;
       
-      _mesa_add_renderbuffer(&b->mesa_buffer, BUFFER_BACK_LEFT,
-                             &b->backxrb->Base.Base);
+      _mesa_add_renderbuffer_without_ref(&b->mesa_buffer, BUFFER_BACK_LEFT,
+                                         &b->backxrb->Base.Base);
    }
 
    /*