3.1.0-ios-fix

This is exactly the same as the official 3.1.0, but with a hotfix to fix linking on iOS for arm64.

It fixes:

Undefined symbols for architecture arm64:
  "_png_init_filter_functions_neon", referenced from:
      _png_read_filter_row in opencv2(pngrutil.o)
ld: symbol(s) not found for architecture arm64