[Generator] Override dispose for opaques

[Therzok]

When owning an opaque, we need to properly implement the Dispose method, otherwise if someone disposes and doesn't wait for the finalizer to hit, it'll leak memory.

https://gist.github.com/Therzok/370fa07bfe84d848fb87cf1d5d3450b9

[Therzok]

Found the actual issue.