Do not override window setting defaults

Created by: endragor

When exporting project settings, Godot only export values not equal to their defaults. Before this fix exporting project from command line omitted display/window/size settings, because their default values were set from existing values in project settings, so Godot considered them unchanged.

This commit adds GLOBAL_DEF_MISSING macro which only overrides the setting's default value if the setting did not already exist.