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/gdnative/videodecoder/video_stream_gdnative.cpp

@@ -279,7 +279,7 @@ void VideoStreamPlaybackGDNative::set_paused(bool p_paused) {
 	paused = p_paused;
 }
 
-Ref<Texture> VideoStreamPlaybackGDNative::get_texture() {
+Ref<Texture> VideoStreamPlaybackGDNative::get_texture() const {
 	return texture;
 }