improve GC handling of out-of-memory

There's a point in attempting to allocate a new large page
where it makes sense to GC and try again.
Mac OS X: incorporate Pango repair for Yosemite

Pango 1.36.6 fixes the problem, so update native libraries
and the Coretext patch.
racket/gui gtk: workaround problem with async IBus input method

Affects Ubuntu 13.10 and 14.04, especially with multiple processors.
ffi/unsafe/objc: fix `objc-is-a?` to recognize subclass instances

This repair turns out to matter for Mac OS X 10.10, where creating
a list box in a frame somehow makes the frame an instance of a
subclass (that implements notifications of some sort, I think).
That subclassing broke an `(objc-is-a? ... RacketWindow)` test to
recognize windows that belong to a particular eventspace.