move adjustment code from sc_adjust_window to adjust_view
This new function adjust_view is in charge of recomputing the scale according to adjustment settings and trigger a render_all. adjust_view contains the old sc_adjust_window code, slightly simplified thanks to the availability of the document_get_viewport_size. Then it is used by sc_adjust_window, document_open and the cb_view_resized callback. Makes slightly more sense this way than calling the shortcut sc_adjust_window directly.