Change Cairo image surface type from RGB24 to ARGB32.

This seems to work around a GTK 3.22 display bug that Colin Watson and
I have both observed on Ubuntu (though I found that proxying the X
server, e.g. by SSH X forwarding or xtruss, inhibited the bug). The
effect of the bug was that the terminal window would appear completely
black and nothing would ever be displayed in it, though the terminal
session was still actually running and keystrokes would be sent to it.

But changing the call to cairo_set_source_surface() to some other
cairo_set_source_foo caused successful drawing of whatever other
source I selected; the problem seemed specific to the image surface.
Also, when I popped up the Ctrl-right-click menu over the terminal
window, the menu didn't disappear when dismissed, i.e. the drawing
area's redraw operation was not drawing in black, but failing to draw
_anything_.

That led me to hypothesise that the draw event handler for the
terminal drawing area might somehow be accidentally inventing 0 rather
than 255 for the implicit alpha channel when using our RGB-type image
surface as a source; so I tried setting the surface type to one with
an explicit alpha channel in the hope that there would no longer be a
need to make up any alpha value at all. And indeed, that seems to
solve the problem for me, so I might as well commit it.

However, I don't know the full details of what the previous problem
was, so this is only an empirical workaround. If it turns out I was
making some other mistake without which a RGB source surface would
have worked for me, then I should probably revert this and do whatever
other fix turns out to be a better plan.

unix/gtkwin.c

@@ -524,7 +524,7 @@ gint configure_area(GtkWidget *widget, GdkEventConfigure *event, gpointer data)
             inst->surface = NULL;
         }
 
-        inst->surface = cairo_image_surface_create(CAIRO_FORMAT_RGB24,
+        inst->surface = cairo_image_surface_create(CAIRO_FORMAT_ARGB32,
                                                    backing_w, backing_h);
 #endif
     }