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.