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/math/convex_hull.cpp

@@ -666,7 +666,7 @@ public:
 		face_pool.reset(true);
 	}
 
-	Vertex *vertex_list;
+	Vertex *vertex_list = nullptr;
 
 	void compute(const Vector3 *p_coords, int32_t p_count);