j3dcore: add type information to userlist in ImageComponentRetained

- no need to synchronize on userlist, all access methods are already synchronized
- add an early return to unindent a bunch of code
- impossible to get a BackGroundRetained here, kill that else branch

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
j3dcore: use a trivial little struct instead of FastVector

Pull out a struct to hold an int[] array and a count, remove the numContours variable that was
redundant.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
j3dcore: remove last use of the BufferWrapper classes from the core

Directly hold read-only views of the nio buffers, this likely fixes the vertex data held in
an nio buffer for the JOGL backend as the Object[] parameter hid the change from float[] to
FloatBuffer that occured at some point in history.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>