[regression] BTextControl: broken horizontal scroll when text don't fit text field

[X512]

This is hrev54752.

This is regression caused by hrev54549.

Steps to reproduce:

1. Open some application with BTextControl.
2. Type someting that don't fit to text control space.
3. Hold left allow key until caret will be at first character.
4. Text will be still scrolled horizontally so text beginning will be at center.
5. When scrolling by pressing and holding mouse button, text can be scrolled to the right so it became not visible at all.

Bug is also present in Tracker file name edit field.