Track managed array pinning in Mat

[Lillenne]

Rationale

Addresses issue #1689 (detailed description there).

Managed arrays are unpinned when the original Mat is disposed or finalized. Captured submat data pointers are not updated on subsequent GC compaction, causing bad data access or AccessViolationExceptions.

Changes

Track GCHandles in the Mat class (via helper class ArrayPinningLifeTime) instead of allowing the DisposeableObject parent class to unpin it when the parent Mat is cleaned up.

Fix #1689

[shimat]

Thank you!!