The main advantage of GTK+ 3 is better support for HiDPI
displays. If GTK+ 3 libraries are not available or if the
`PLT_GTK2` environment variable is defined, GTK+ 2 is used
as before.
Allocation rules:
* Use `as-objc-allocation' when creating a Cocoa object. When the
resulting reference becomes unreachable, the Cocoa object will be
released.
* Use `with-autorelease' in atomic mode around calls that autorelease
and where the release should take effect immediate. Do not create
an autorelease pool except in atomic mode.
* Other autoreleased objects may end up in the root pool installed by
"pool.rkt". The root pool is periodically destroyed and replaced;
call `queue-autorelease-flush' if you need to encurage replacement
of the pool. If you need to use an object that might be autoflushed,
be sure that you're in atomic mode.
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