Film implementations: don't write bitmap output files if destFile=''

src/films/hdrfilm.cpp

@@ -398,6 +398,9 @@ public:
 	}
 
 	void develop(const Scene *scene, Float renderTime) {
+		if (m_destFile.empty())
+			return;
+
 		Log(EDebug, "Developing film ..");
 
 		ref<Bitmap> bitmap = m_storage->getBitmap()->convert(

src/films/ldrfilm.cpp

@@ -298,6 +298,9 @@ public:
 	}
 
 	void develop(const Scene *scene, Float renderTime) {
+		if (m_destFile.empty())
+			return;
+
 		Log(EDebug, "Developing film ..");
 
 		ref<Bitmap> bitmap = m_storage->getBitmap();

src/films/mfilm.cpp

@@ -239,6 +239,9 @@ public:
 	}
 
 	void develop(const Scene *scene, Float renderTime) {
+		if (m_destFile.empty())
+			return;
+
 		Log(EDebug, "Developing film ..");
 
 		fs::path filename = m_destFile;