Extracted fields for delegates in BindCallbacks

[theraot]

Since the GC is not aware that the extern calls hold reference to passed delegates, it will collect these delegates which will result in some nasty exceptions※. To prevent this, I have extracted the delegates as fields, which will be tied to the lifetime of the NativeWindow object (making sure the GC does not collect them before collecting the NativeWindow object).

※: I was getting ExecutionEngineException on .NET Core and CallbackOnCollectedDelegate on .NET Framework on calls to Glfw.PollEvents.

I have tested that my code works with builds of glfw-net that include this change and the problem has not come back.

Note: I had missed SetErrorCallback the first time around.

[ForeverZer0]

Good catch, my primary testing ground has been .NET Core on Arch Linux, I apparently missed that test with the .NET Framework.

I will likely create a new release later today with the merged changes.