fix warnings in samples

samples/cpp/cloning_gui.cpp

@@ -65,7 +65,7 @@ float alpha,beta;
 
 float red, green, blue;
 
-double low_t, high_t;
+float low_t, high_t;
 
 void source(int, int, int, int, void*);
 void destination(int, int, int, int, void*);

samples/cpp/tutorial_code/photo/seamless_cloning/cloning_gui.cpp

@@ -64,7 +64,7 @@ float alpha,beta;
 
 float red, green, blue;
 
-double low_t, high_t;
+float low_t, high_t;
 
 void source(int, int, int, int, void*);
 void destination(int, int, int, int, void*);