add dispose() method to PixelData
use dispose() for ROI PixelData
use dispose() for projections
use dispose() in raw pixels bean
use dispose() in pixels service
use dispose() where practical in romio pixel buffer
add extra note on use of dispose()
Merge pull request #1865 from mtbc/trac-11675-leaking-direct-buffers

assist #11675: prevent some leaking direct buffers