[IOS] burning subtitles fails on arm64

[tanersener]

As detected during tests of v2.1, and noted in https://github.com/tanersener/mobile-ffmpeg/wiki/Known-Issues, burning subtitles crashes for IOS arm64 architecture. This crash is specific to arm64 and comes from libavfilter itself. A fast solution is not possible and a further analysis is required to build a fix.

[tanersener]

A detailed analysis shows that crash is caused by EXC_BAD_ACCESS signal, which is thrown when unlock_cache() function from src/fccache.c file of fontconfig library is accessed.

static void
unlock_cache (void)
{
  FcMutexUnlock (cache_lock);
}

Initialization of cache_lock static field is the actual problem here. A fix is needed for that.

[tanersener]

Fixed in v2.1.1 IOS patch.