Fix leaks caused by implicitly-called copy constructors

[sheepandshepherd]

Previously, Variants were unpacked like this:

Variant::operator PoolVector2Array() const {
	godot_pool_vector2_array s = godot::api->godot_variant_as_pool_vector2_array(&_godot_variant);
	return *(PoolVector2Array *)&s;
}

The cast implicitly calls the C++ type's copy constructor. (You can find it in the gdb disassembly.) This increases the ref-count twice instead of just once through godot_variant_as_*. For PoolArrays, it caused an error message on exiting Godot:

ERROR: cleanup: There are still MemoryPool allocs in use at exit!
   At: core/pool_vector.cpp:69.

All of the ref-counted, RAII-managed core types are affected. The fix is to use a local variable and return it instead of casting.

[zhangshiqian1214]

I test this fix is ok and can fix #318, your solution is beautiful, only use a local variable fix ref-count

[akien-mga]

Thanks!

[sheepandshepherd]

Looks like this still leaks for Array and Dictionary. I forgot to take into account that their default constructors, unlike PoolArrays, allocate a small empty array that is now getting leaked instead of the one in the Variant.

I don't know of any way to prevent this default constructor call in C++. Instead of trying more hackish casts, I think I'll just add a safe, clean constructor to Array and Dictionary: Array(godot_array p_array), unless anyone has another idea for how to convert the C types without calling either default or copy constructor.