Style: Partially apply clang-tidy's cppcoreguidelines-pro-type-member-init

Didn't commit all the changes where it wants to initialize a struct
with `{}`. Should be reviewed in a separate PR.

Option `IgnoreArrays` enabled for now to be conservative, can be
disabled to see if it proposes more useful changes.

Also fixed manually a handful of other missing initializations / moved
some from constructors.

modules/theora/video_stream_theora.h

@@ -99,7 +99,7 @@ class VideoStreamPlaybackTheora : public VideoStreamPlayback {
 
 	Ref<ImageTexture> texture;
 
-	AudioMixCallback mix_callback;
+	AudioMixCallback mix_callback = nullptr;
 	void *mix_udata = nullptr;
 	bool paused = false;