PDFViewer: Support a continuous page view mode

Author: https://github.com/mattco98
Commit: 3ecb41b7d9
Pull-request: https://github.com/SerenityOS/serenity/pull/13394
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/nico

Userland/Libraries/LibPDF/Document.h

@@ -23,6 +23,9 @@ struct Rectangle {
     float lower_left_y;
     float upper_right_x;
     float upper_right_y;
+
+    float width() const { return upper_right_x - lower_left_x; }
+    float height() const { return upper_right_y - lower_left_y; }
 };
 
 struct Page {