stb_image's vertical flip flag was not thread safe in the version
of stb_image we were using. We patched stb_image to use a thread
local variable for the flag. stb_image has since been upgraded to
expose a thread local version of the flag, so our patch is no longer
necessary after upgrading.
The CMake flag to enable the thread local patch did not make very much
sense because thread local stuff is unconditionally used elsewhere.