vsgiosnative example

[iconiqlabs]

added an example of VulkanSceneGraph running in iOS

[robertosfield]

@iconiqlabs I merged prematurely. While the merge with VulkanSceneGraph master when smoothly and when I merged the changes to vsgExamples and built under Linux and get errors due all the iOS specific CMake code placed into vsgExamples/examples/platform/CMakeLists.txt.

I don't what how it should be done, but I do know that it won't be done by putting iOS specific cmake into a general CMakeLists.txt file, these iOS specific cmake entriies should go into examples/platform/vsgiosnative directly and have a conditionally compiled check in the like it already has for Android:

if (ANDROID)
add_subdirectory(vsgandroidnative)
endif()