Make util/geometry.h C++20 friendly

- Move header to base/include to make it applicable for base types
  like 'Affinity' down the road.
- Represent 'Rect' as typle of point and area, which is the most
  common form of initialization, creates in valid 'Rect' by default.
- Turn Point, Area, and Rect into compound types, making x, y, w, h, at,
  area accessible without a method call
- 'Rect::Compound' function for constructing a 'Rect' from two points,
  replacing a former constructor
- Use result type 'Rect::Cut_remainder' instead of out parameters.

Fixes #5239

repos/demo/include/scout/element.h

@@ -88,8 +88,8 @@ class Scout::Element
 		 */
 		virtual void geometry(Rect rect)
 		{
-			_position = rect.p1();
-			_size     = rect.area();
+			_position = rect.at;
+			_size     = rect.area;
 		}
 
 		/**
@@ -184,7 +184,7 @@ class Scout::Element
 		/**
 		 * Trigger the refresh of an element on screen
 		 */
-		void refresh() { redraw_area(0, 0, _size.w(), _size.h()); }
+		void refresh() { redraw_area(0, 0, _size.w, _size.h); }
 
 		/**
 		 * Handle user input or timer event