Made gui2::point implicitly convertible to SDL_Point

I'm planning on moving this out of the GUI2 namespace and this will allow point objects
to be used with SDL functions that take SDL_Point arguments.

src/gui/core/point.cpp

@@ -20,6 +20,10 @@
 
 namespace gui2
 {
+point::operator SDL_Point()
+{
+    return {x, y};
+}
 
 point& point::operator+=(const point& point)
 {

src/gui/core/point.hpp

@@ -14,6 +14,8 @@
 
 #pragma once
 
+#include <SDL_rect.h>
+
 #include <iosfwd>
 
 namespace gui2
@@ -36,6 +38,9 @@ struct point
 	/** y coordinate. */
 	int y;
 
+	/** Allow implicit conversion to SDL_Point. */
+	operator SDL_Point();
+
 	bool operator==(const point& point) const
 	{
 		return x == point.x && y == point.y;