Simpler implementation of Fl_GDI_Graphics_Driver::copy_offscreen_with_alpha() does just as well.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10334 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
Removed useless extern void fl_restore_clip()

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10335 ea41ed52-d2ee-0310-a9c1-e6b18d33e121