[Lazarus] repaint method on Darwin/Carbon

[dmitry boyarintsev]

> 1. like dmitry says - Carbon/Cocoa & Qt on all platforms does not allow
> paintings outside of paint event.
> 2. Calling repaint is very expensive (CPU), try with YourControl.Invalidate;
> or YourControl.Update;

Invalidate and Update will take effect only if messages queue is
processed (by ProcessMessages or PostMessages if blocked by an event).