autoScroll implementation is wrong

Changing autoScroll does not work properly, because the TextInput hscroll is calculated in the paint event and it is relative to previous values. This is very wrong. It makes it impossible to publish the value as a property, makes it not possible to test sensibly, and means that the value does not always update in a single frame.

The calculation of hscroll should be done when needed (eg. font change, text change, cursor position changed, autoscroll changed), or the feature should just be removed and a simple example provided of how to implement it oneself.