Convenience method for serializing OBJ files

<h2>Why

In making my applications I have religiously serialized all of my OBJ files. Because it is a bit of a laborious process I whipped up this quick convenience method for outputting OBJ files to external storage.

<h2>Where

RajawaliRenderer seemed like the best place for this method so it could be easily accessed during app creation.

<h2> How

Simple, there are four options:

serializeObj(int resourceId, String outputName)
serializeObj(int resourceId, String outputName, Boolean compress)
serializeObj(int resourceId, String outputName, File exportDir)
serializeObj(int resourceId, String outputName, Boolean compress, File exportDir)

"exportDir" is the directory path for MeshExporter.