Don't process events handled in user code in generic scrolling code.

Undo the change introduced, probably accidentally, by r11369 (12 years ago)
and skip the default handling of all events except wxEVT_SIZE and wxEVT_PAINT,
which are special for the reasons explained in the comments in the code, if
the user code has already handled the event.

This allows to customize scrolling by selectively handling some scrolling
events only and generally makes sense.

Closes #15684.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75252 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775