Another run at the problem 57516164de was meant to solve. The
new solution just gives up when a `drawRect:' method is called
in the wrong thread, which might create refresh glitches but
avoids a deadlock in the case that the Racket thread is blocked
on the update happening in the foreign thread.
On Cocoa, a view's `drawRect:' method can be called from a
heartbeat thread that animates controls. Such a call happens
rarely for a `canvas%' or other class where `drawRect:'
is overridden, but since it can happen, ensure that the
callback runs on the Racket thread.
"My" -> "Racket". The "My" prefix was from initial experiments,
of course, and I just never got around to changing it before.
I think these names go into a global namespace, though, at the
ObjC level, so they need to have a distinct and Racket-specific
prefix.
