switch from fopen to fopen_s on later MSVCs

Proper pointer check before deleting

corrected annoying warnings about 'missing initializer'. The memset
version is not better, but gcc like it more.

Change byte casts in stb_image.h that intentionally truncate to do so explicitly for MSVC runtime check compatibility

Fix handling of iphone-procesed image to not accidentally appear corrupt (instead they just load wrong).
Add a proper testing path to image test