Ligatures ignored on input on Windows

If a keyboard layout defines ligatures, GTK+ applications behave as if the modifier key used to produce the ligature wasn't pressed.

Steps to reproduce

1. Install https://eternallybored.org/misc/keyb/engtest.exe - this will add "English (international) - test keyboard" layout to Windows (if it doesn't appear in the layout picker immediately, it may have to be installed through the "Add a language button" in Settings -> Time and language -> Region & language)
2. Open any GTK+2 or GTK+3 program
3. Go to any text entry field, make sure that "English (international) - test keyboard" is selected and press AltGr+B or AltGr+G (AltGr is right Alt key).

Current behavior

The letters b and g appear as if AltGr wasn't held down.

Expected outcome

Pressing AltGr+B should result in the text "Test" to be inserted, and AltGr+G should produce the 👍 emoji.

Version information

Tested with GTK+ 2.24.32 (gtk-demo.exe) and GTK+ 3.24.1 (gtk3-demo-application.exe) on Windows 10 1803; GTK+ from the MSys2 distribution.