Fixes a GDI bug dating back to July 2007.

For some reason the DoFreeImage call was commented and thus Windows
apps leaked GDI objects. Once the 9000 limit was hit, paint corruption
occured.

Many thanks to Hinst <hinstance@yandex.ru> for persisting and finding
this bug.
Removed redundant code in TfpgScrollbar.

The Canvas.BeginDraw/EndDraw calls are not needed. The painting outside
the HandlePaint() event was long ago fixed, but these calls were simply
not removed.