GtkMagnifier: Add a resizing mode

GtkMagnifier: Add a resizing mode

In addition to the fixed-size mode that is used by the text view,
add a mode in which the magnifier requests enough size to render
the entire inspected widget at the current magnification. In this
mode, the magnifier will update its size when the size of the
inspected widget changes. Also, make the magnifier redraw on its
own whenever the inspected widget draws.