Changes

MacOSX: Fix shared ctx release [onMainThread]; Make GLContextShareSet lifecycle deterministic; Remove warnings

Fix shared ctx release [onMainThread]
  - Releasing the shared contexts caused a freeze of about 10s from one of the shared release operations.
    [NSOpenGLContext release]
  - Thorough triage concluded the workaround to release the shared ctx on the main thread.
  - Using enhanced GLContextShareSet, see below

Make GLContextShareSet lifecycle deterministic
  - Programmatically control the lifecycle of tracked shared ctx allows us using 'hard' references.
  - Features queries for isShared() and ofc unregister a share set if all are destroyed.

Remove warnings
  - MacOSXWindowSystemInterface.m used 'long', where 'GLint' was requested.