The origin of corresponding widget should be on the same x position
as combo's button. But it isn't.

[Steps to Reproduce]
1. Run testgtk program in ./tests on a BiDi locale (e.g. he_IL)
2. Press `entry' button.
3. Click on combo's button to display drop-down list.
   The position of drop-down list isn't on the same x position
   as combo's button. <-- Problem

[Actual Results]
Origin of corresponding widget is in the same x positions as combo's
entry field.

[Expected Results]
Origin of corresponding widget should be on the same x position as
combo's button.