Catch NULL results from CoreFoundation/CoreText creation functions and
return error.OOM rather than null derefs later. I verified that this is
possible but didn't verify the behavior when it happens, this is just
defensive based on the report here: #13671 because it costs us nothing
really.
Some CoreFoundation objects, such as those produced by CoreText, have
expensive callbacks that run when they are released. By offloading the
CFRelease calls to another thread, we can avoid important threads being
blocked by unexpectedly expensive callbacks.
This commit also changes the way that the coretext shaper's run iterator
builds its string. Rather than using a CFMutableString, an ArrayList of
unichars is built which is passed to CFStringCreateWithCharactersNoCopy,
which is a lot more efficient since it avoids all the CoreFoundation
overhead.
