Enhance Fl_Rect: add inset() and new constructor

These new features can be used by widgets that draw inside a rectangle
taking the border width into account.

FL/Fl_Rect.H

@@ -1,7 +1,7 @@
 //
 // Fl_Rect header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2018 by Bill Spitzak and others.
+// Copyright 1998-2022 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -50,6 +50,17 @@ public:
   Fl_Rect(int X, int Y, int W, int H)
           : x_(X), y_(Y), w_(W), h_(H) {}
 
+  /** This constructor creates a rectangle with the given x,y coordinates
+    and the given width and height reduced by the box frame size.
+
+    This is the same as using the constructor w/o \p bt and subsequently
+    calling inset(\p bt).
+  */
+  Fl_Rect(int X, int Y, int W, int H, Fl_Boxtype bt)
+          : x_(X), y_(Y), w_(W), h_(H) {
+    inset(bt);
+  }
+
   /** This constructor creates a rectangle based on a widget's position and size. */
   Fl_Rect (const Fl_Widget& widget)
           : x_(widget.x()), y_(widget.y()), w_(widget.w()), h_(widget.h()) {}
@@ -77,6 +88,43 @@ public:
   void w(int W) { w_ = W; }             ///< sets the width
   void h(int H) { h_ = H; }             ///< sets the height
 
+  /** Move all edges in by \p d.
+
+    Shrinks the rectangle by \p d at all sides keeping the center of the
+    rectangle at the same spot.
+
+    If \p d is negative, the rectangle is enlarged.
+
+    If \p d \>= w() or h() the result is undefined, i.e. an
+    invalid or empty rectangle.
+  */
+  void inset(int d) {
+    x_ += d;
+    y_ += d;
+    w_ -= 2 * d;
+    h_ -= 2 * d;
+  }
+
+  /** Move all edges in by the frame size of box type \p bt.
+
+    Shrinks the rectangle at all sides by the frame width or height of the
+    given box type \p bt.
+
+    This method uses the frame sizes given by the box type \p bt using
+    - Fl::box_dx(bt)
+    - Fl::box_dy(bt)
+    - Fl::box_dw(bt)
+    - Fl::box_dh(bt)
+
+    If the rectangle is smaller than the frame sizes the result is undefined,
+    i.e. an invalid or empty rectangle.
+  */
+  void inset(Fl_Boxtype bt) {
+    x_ += Fl::box_dx(bt);
+    y_ += Fl::box_dy(bt);
+    w_ -= Fl::box_dw(bt);
+    h_ -= Fl::box_dh(bt);
+  }
 }; // class Fl_Rect
 
 #endif // Fl_Rect_H