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.

core/input/input.h

@@ -113,10 +113,10 @@ private:
 	int mouse_from_touch_index = -1;
 
 	struct VelocityTrack {
-		uint64_t last_tick;
+		uint64_t last_tick = 0;
 		Vector2 velocity;
 		Vector2 accum;
-		float accum_t;
+		float accum_t = 0.0f;
 		float min_ref_frame;
 		float max_ref_frame;