Windows fix - #213

Note: to work perfectly, the fix of GD Converter must be considered:
rosell-dk/webp-convert#113

Thanks a lot!

I predict that webp-realizer.php isn't working on Windows? - It is the ability to generate a webp when the webp is visited. It now calls ConvertHelperIndependent::findSourceSeparate to find source. And that currently uses the following code to decide if $destination is inside the "doc-root" folder of the cache dir:

if (strpos($destination, $imageRoot . '/doc-root/') === 0) {
   ...
}

We cannot use realpath in this case, because $destination does not exist yet.

Yes, you are right, you can't use realpath to normalize directory paths because it only works with real paths (paths that exist in current filesystem) as the name of function says, but you can create a function that does that or get a function from a composer module that handle this, it would be interesting look for some module that works both for Linux, Windows and related systems.