updated project.xml for the Shooter tutorial

....disabled window resizing, changed the background color, and, most
importantly, I added an asset path to the assets/atlas directory
create an atlas for image resources (.png, etc)

create an assets/atlas/ directory and place an atlas.xml file in it.
This is where we will define the regions of the atlas which represent
different graphics. Once again, if you use a program like TexturePacker,
this xml will be generated automatically