c++: Turn (Logical|Physical)(Size|Position)` into structs

They used to declared by `using` before, so they were too easy to
convert into.

api/cpp/include/slint_point.h

@@ -22,15 +22,27 @@ struct Point
 };
 
 namespace cbindgen_private {
-// The Size types are expanded to the Point2D<...> type from the euclid crate which
+// The Point types are expanded to the Point2D<...> type from the euclid crate which
 // is binary compatible with Point<T>
 template<typename T>
 using Point2D = Point<T>;
 }
 
 /// A position in logical pixel coordinates
-using LogicalPosition = Point<float>;
+struct LogicalPosition : public Point<float>
+{
+    /// Explicitly convert a Point<float> to a LogicalPosition
+    explicit LogicalPosition(const Point<float> p) : Point<float>(p) {};
+    /// Default construct a LogicalPosition in the origin
+    LogicalPosition() : Point<float> { 0., 0. } {};
+};
 /// A position in physical pixel coordinates
-using PhysicalPosition = Point<int32_t>;
+struct PhysicalPosition : public Point<int32_t>
+{
+    /// Explicitly convert a Point<int32_t> to a LogicalPosition
+    explicit PhysicalPosition(const Point<int32_t> p) : Point<int32_t>(p) {};
+    /// Default construct a PhysicalPosition in the origin
+    PhysicalPosition() : Point<int32_t> { 0, 0 } {};
+};
 
 }