ffi/unsafe/obj: add `+A' and `-A' method modes

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.
Complain visibly when cannot load GL shared libs, instead of swallowing error.
ffi/unsafe: add `_double' and `_float' tests
upgrade libffi to 3.0.13

This upgrade is particularly for repairs to ARM and the VFP ABI.