Fix Coverity reports of uninitialized scalar variable

Fixes most current reports on Coverity Scan of uninitialized scalar
variable (CWE-457): https://cwe.mitre.org/data/definitions/457.html

These happen most of the time (in our code) when instanciating structs
without a constructor (or with an incomplete one), and later returning
the instance. This is sometimes intended though, as some parameters are
only used in some situations and should not be double-initialized for
performance reasons (e.g. `constant` in ShaderLanguage::Token).

core/math/quick_hull.cpp

@@ -74,7 +74,7 @@ Error QuickHull::build(const Vector<Vector3> &p_points, Geometry::MeshData &r_me
 	int longest_axis = aabb.get_longest_axis_index();
 
 	//first two vertices are the most distant
-	int simplex[4];
+	int simplex[4] = { 0 };
 
 	{
 		real_t max = 0, min = 0;