add macOS implementation

macos/include/StreamBufferAdapter.hpp

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <streambuf>
+#include <functional>
+#include <cassert>
+#include <cstring>
+
+namespace polyvox {
+
+  using namespace std;
+
+  // 
+  // A generic adapter to expose any contiguous section of memory as a std::streambuf.
+  // Mostly for Android/iOS assets which may not be able to be directly loaded as streams.
+  //
+  class StreamBufferAdapter : public std::streambuf
+  {
+      public:
+          StreamBufferAdapter(const char *begin, const char *end);
+          ~StreamBufferAdapter() {
+            
+          }
+          streamsize size();
+          
+      private:
+          int_type uflow() override;
+          int_type underflow() override;
+          int_type pbackfail(int_type ch) override;
+          streampos seekoff(streamoff off, ios_base::seekdir way, ios_base::openmode which) override;
+          streampos seekpos(streampos sp, ios_base::openmode which) override;
+          streamsize showmanyc() override;
+          
+  };
+
+}