NVIZ debug tip added (merge devbr6, https://trac.osgeo.org/grass/changeset/31567)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@31568 15284696-431f-4ddb-bdfa-cd5b030d7da7

doc/debugging.txt

@@ -50,6 +50,12 @@ NVIZ debug tip:
    at the start of nviz2.2_script, change the DEBUG setting to:
    set DEBUG 1
 
+To debug NVIZ using gdb:
+
+$ gdb $GISBASE/etc/nviz2.2/nviz
+> attach <pid>
+> break Create_OS_Ctx
+> cont
 
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