Fix compile failure on MacOS X

std::shared_ptr::unique() is deprecated in C++17 and removed in C++20.

Inspired by PR#1362.

Snippet of the failure:

```
In file included from ../../src/terminal/terminal.h:42:
../../src/terminal/terminalframebuffer.h:431:23: error: 'unique' is deprecated [-Werror,-Wdeprecated-declarations]
  431 |     if ( !mutable_row.unique() ) {
      |                       ^
/Applications/Xcode_16.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__memory/shared_ptr.h:752:3: note: 'unique' has been explicitly marked deprecated here
  752 |   _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_HIDE_FROM_ABI bool unique() const _NOEXCEPT { return use_count() == 1; }
      |   ^
/Applications/Xcode_16.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__config:731:41: note: expanded from macro '_LIBCPP_DEPRECATED_IN_CXX17'
  731 | #    define _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_DEPRECATED
      |                                         ^
/Applications/Xcode_16.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__config:696:49: note: expanded from macro '_LIBCPP_DEPRECATED'
  696 | #      define _LIBCPP_DEPRECATED __attribute__((__deprecated__))
      |                                                 ^
1 error generated.
```