create texture path double slash fix

extractUrlBase already add a slash on texturePath hence an error occurs as for instance if texturePath is "/folder/folder2/" and sourceFile is "my.jpg" the result will be /folder/folder2//my.jpg.

It's a common issue in file path. For instance Java uses new File(String path, String fileName) constructor to avoid all those tricky things such as Windows/linux slashes and ending slash, unfortunatelly it does not exists in JS ;)