Codechange: Move RectPadding::zero definition to header. (#11461)

This potentially allows better inlining/optimization.

src/core/geometry_func.cpp

@@ -48,5 +48,3 @@ Rect BoundingRect(const Rect &r1, const Rect &r2)
 
 	return r;
 }
-
-const RectPadding RectPadding::zero = {0, 0, 0, 0};

src/core/geometry_type.hpp

@@ -65,6 +65,8 @@ struct RectPadding {
 	inline uint Vertical() const { return this->top + this->bottom; }
 };
 
+inline const RectPadding RectPadding::zero{};
+
 /** Specification of a rectangle with absolute coordinates of all edges */
 struct Rect {
 	int left;