Pointers to native classes should _not_ be dereferenced by C#

[AArnott]

Actual behavior

The GdipDeleteRegion method gets a friendly overload that takes ref GpRegion instead of GpRegion*.
This might be fine, except that GpRegion* should never be dereferenced, since it's a pointer to a class on the C++ side. The struct defined in the metadata has an IntPtr field, but unlike HWND where that is just a copyable opaque value, GpRegion (and other GDI+ structs) actually represent C++ classes, and the 'field' in these structs is misleading. What would really be at that memory location is the first field in the C++ class, so copying around the struct in C# would lead to data corruption as anyone that re-takes the pointer to the struct would expect the full class fields where the struct is, and they wouldn't be there.

Expected behavior

Any extern methods that take a pointer to a native object should not have friendly overloads that replace the pointer with ref or other modifiers.

[AArnott]

Blocked on microsoft/win32metadata#1821