opengl renderer: create rendering subwindow

The application provides the window handle to which the
OpenglRenderer should render to however only a sub-region of
this window needs to be rendered. This change adds this functionality
by creating a native child subwindow into which rendering will happen.

tools/emulator/opengl/host/include/libOpenglRender/render_api.h

@@ -31,6 +31,10 @@ typedef HWND    FBNativeWindowType;
 
 typedef Window   FBNativeWindowType;
 
+#elif defined(__APPLE__)
+
+typedef void*   FBNativeWindowType;
+
 #else
 #warning "Unsupported platform"
 #endif