fix a number of memory leaks

- some were with libav functions
- same were due to non-virtual destructors

src/ClipBase.cpp

@@ -29,6 +29,9 @@
 
 using namespace openshot;
 
+ClipBase::~ClipBase() {
+}
+
 // Generate Json::JsonValue for this object
 Json::Value ClipBase::JsonValue() {
 
@@ -108,4 +111,4 @@ Json::Value ClipBase::add_property_choice_json(string name, int value, int selec
 
 	// return JsonValue
 	return new_choice;
-}
+}