From Builder, we need a way to start a program with a given text direction so that we can simplify the process for developers to test GTK applications with settings different than their native locale.
This adds a "GTK_DEBUG_TEXT_DIR" environment variable which may be set to either rtl or ltr. DEBUG is used in the name to make it clear that this environment variable is meant for that case only, not as API which can be relied on for workarounds.
Related to gnome-builder#935
If this is desirable, it would be nice to backport it to GTK 3 as well since Builder still supports creating and running "legacy" GTK 3 applications.