Removed one use of dynamic_cast.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10480 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
Improved handling of high resolution ("retina") displays.
The new field bool Fl_X::mapped_to_retina is updated when needed, that is, when the window is moved.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10481 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
Propagate Fl_X::mapped_to_retina value from parent to subwindows.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10482 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
Fix case when show() of a subwindow whose parent is not shown yet.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10483 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
It is necessary to compute Fl_X::mapped_to_retina after the window is mapped
because the windowDidMove notification is not always sent.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10484 ea41ed52-d2ee-0310-a9c1-e6b18d33e121