examples/core_usb_demo: Move player sprite to loop()

Don't have too many global variables.
examples/core_usb_demo: Initialize camera in draw()
examples/core_usb_demo: Add comments to loop variables
core_usb_demo: Set sprite depth in draw()
core_usb_demo: Remove player sprite structure

It only has a single instance so why bother?
core_usb_demo: Replace for loop with while loop