OpenGL context sharing
ffi/unsafe/obj: fix use of ivar in rator position

Merge to v5.3.4
racket/gui cocoa: delay `on-paint' if a GL context isn't ready

The relevant `on-paint' call is one that is forced for a GL
canvas to try to draw a frame's content before the frame is
shown. Sometimes, thread scheduling would let the frame get
sufficiently initialized at the Cocoa level before the `on-paint'
call happened, but sometimes not.
racket/enter: fix exception handling & propagation for `enter!'

Merge to v5.3.4