Refactor VideoPlayer and VideoStream

VideoStream:
- Fix const correctenss

VideoPlayer:
- Remove unused member variable last_frame
- Move _mix_audios function definition to source file
- Fix function parameter naming to match p_ convention
- Fix const correctness
- Add null checking

modules/theora/video_stream_theora.cpp

@@ -368,7 +368,7 @@ float VideoStreamPlaybackTheora::get_time() const {
 	return time - AudioServer::get_singleton()->get_output_latency() - delay_compensation; //-((get_total())/(float)vi.rate);
 };
 
-Ref<Texture> VideoStreamPlaybackTheora::get_texture() {
+Ref<Texture> VideoStreamPlaybackTheora::get_texture() const {
 
 	return texture;
 }

modules/theora/video_stream_theora.h

@@ -147,7 +147,7 @@ public:
 
 	void set_file(const String &p_file);
 
-	virtual Ref<Texture> get_texture();
+	virtual Ref<Texture> get_texture() const;
 	virtual void update(float p_delta);
 
 	virtual void set_mix_callback(AudioMixCallback p_callback, void *p_userdata);