UI: Change cursor when interacting with the preview

Includes move, resize, crop, stretch, box select.

UI/window-basic-preview.hpp

@@ -92,6 +92,8 @@ private:
 
 	void GetStretchHandleData(const vec2 &pos);
 
+	void UpdateCursor(uint32_t &flags);
+
 	void SnapStretchingToScreen(vec3 &tl, vec3 &br);
 	void ClampAspect(vec3 &tl, vec3 &br, vec2 &size, const vec2 &baseSize);
 	vec3 CalculateStretchPos(const vec3 &tl, const vec3 &br);