Cleanup: improve 2D convex hull

Improve correctness, API, comments, memory usage and performance
of the 2D convex hull calculation.

Pre-requisite for UV packing improvements.

Differential Revision: https://developer.blender.org/D16055

source/blender/python/mathutils/mathutils_geometry.c

@@ -1465,7 +1465,7 @@ static PyObject *M_Geometry_convex_hull_2d(PyObject *UNUSED(self), PyObject *poi
     int *index_map;
     Py_ssize_t len_ret, i;
 
-    index_map = MEM_mallocN(sizeof(*index_map) * len * 2, __func__);
+    index_map = MEM_mallocN(sizeof(*index_map) * len, __func__);
 
     /* Non Python function */
     len_ret = BLI_convexhull_2d(points, len, index_map);