Add const to InputEventMouseButton::get_factor

(cherry picked from commit 982efb186404c35ac954ac5e14a83231ade8f13c)

core/os/input_event.cpp

@@ -416,7 +416,7 @@ void InputEventMouseButton::set_factor(float p_factor) {
 	factor = p_factor;
 }
 
-float InputEventMouseButton::get_factor() {
+float InputEventMouseButton::get_factor() const {
 
 	return factor;
 }

core/os/input_event.h

@@ -328,7 +328,7 @@ protected:
 
 public:
 	void set_factor(float p_factor);
-	float get_factor();
+	float get_factor() const;
 
 	void set_button_index(int p_index);
 	int get_button_index() const;