Lwjgl3: Replace Unsafe-based object scanning with a spec-compliat mechanism.

[mukel]

Environment

• LWJGL version: 3.3.0 SNAPSHOT
• LWJGL build #: X
• Java version: ALL
• Platform: Linux|macOS|Windows
• Module: core

Description

To find private field offsets of DirectByteBuffer, objects are scanned using Unsafe e.g. see MemoryUtil#getFieldOffset

Scanning objects like that is very fragile, field offsets are just a token/cookie, the physical location remains an implementation detail. The JVM may decide the shuffle fields (packing), or even remove fields that are always constant or unused and provide a "fake" offset instead.
The issue was originally reported/found here. Espresso is an alternative, spec-compliant JVM; it uses the OpenJDK class library but an different object model/layout that is not compatible with this pattern.

A spec-compliant improvement would be to scan all fields (instead of all offsets) while searching for a magic constant. This should be more resilient to object layout changes and alignment issues.

[Spasi]

Thanks @mukel!