add double_buffering options in fbcon section in MiniGUI.cfg

etc/MiniGUI.cfg

@@ -105,6 +105,12 @@ seat=seat0
 [fbcon]
 defaultmode=1024x768-16bpp
 dpi=96
+
+# Whether double buffering is enabled (only for threads and standalone runmodes).
+# Use `true` or `yes` for enabled, anything else for disabled.
+# Note that under compositing schema, double buffering is always enabled.
+# Since 5.0.10.
+double_buffering=true
 #}}
 
 #{{ifdef _MGGAL_PCXVFB