Don't reuse the same list for _get_property_list()

[dsnopek]

In order to work around an issue where we need to keep the List<PropertyInfo> for _get_property_list() alive until the data it contains isn't needed anymore, we've been reusing the same List<PropertyInfo>.

This works fine in a single-threaded scenario, but when multiple threads are involved, they'll stomp all over the list at the same time.

This PR makes it so that we aren't reusing the same List<PropertyInfo>, but instead, allocating a new one each time.

The tricky part is that we need to hang onto a pointer to it, so that we can free it when we're done. However, Godot will only pass us the GDExtensionPropertyInfo * and doesn't let us stash any extra data. So, this PR uses the padding that's added by Memory::alloc_static() to stash the pointer in the same place that we usually put the element count when doing memnew_arr().

This is a little hacky, and it would be better if Godot gave us a place to stash some extra data instead, but this does seem to work in my testing :-)

Fixes godotengine/godot#115017

[bruvzg]

Fixes the issue, code looks good.

[akien-mga]

Thanks!