"Hatran is written from right to left horizontally"

http://www.unicode.org/L2/L2012/12312-n4324-hatran.pdf

This ancient script was added with Unicode 8.0.

Also this spreadsheet (referenced in an inline comment in the
source code) has it as RTL: http://goo.gl/x9ilM

Fixes https://github.com/harfbuzz/harfbuzz/issues/711

Really, need better way to catch these :(.

Fixes https://oss-fuzz.com/v2/testcase-detail/5216838347653120
which is a stack overflow, not by way of infinite recursion, just
being deep.  That's disallowed anyway, so catch it as it happens,
not afterwards.

I hate it that with my compiler at least, it doesn't catch totally uncompilable
statements in templates if they are not used...

Not sure why only one of the bots sees this as visible...

To better catch errors locally.

Maybe I should disable C++11 detection such that my compiler also catches
these.  Doesn't look like we can switch to it for a while anyway...

Defining _POSIX_C_SOURCE to an old version on FreeBSD can cause C99 to
be disabled in libc.

Ouch. I need a better workflow for this.

It's dead, Jim!

Apparently CoreText does allow these.  To be done: detect infinite
loops.

Fixes MORX-12 test.