File operation wrapping

Use these helper functions if you want to access a file from the perspective of a specific wsi, which is usually the case. If you just want contextless file access, use the fops callbacks directly with NULL wsi instead of these helpers.

If so, then it calls the platform handler or user overrides where present (as defined in info->fops)

The advantage from all this is user code can be portable for file operations without having to deal with differences between platforms.