Describe Cell width as a narrow/wide boolean.

A character cell can only be either narrow or wide.  It's more
convenient to represent that as an int containing 1 or 2, but slightly
more correct to represent it as a "boolean" single-bit integer.

src/frontend/terminaloverlay.cc

@@ -280,7 +280,7 @@ void NotificationEngine::apply( Framebuffer &fb ) const
       this_cell->get_renditions().background_color = 44;
       
       this_cell->append( ch );
-      this_cell->set_width( chwidth );
+      this_cell->set_wide( chwidth == 2 );
       combining_cell = this_cell;
 
       overlay_col += chwidth;
@@ -291,7 +291,7 @@ void NotificationEngine::apply( Framebuffer &fb ) const
       }
 
       if ( combining_cell->empty() ) {
-	assert( combining_cell->get_width() == 1 );
+	assert( !combining_cell->get_wide() );
 	combining_cell->set_fallback( true );
 	overlay_col++;
       }