assist #11675: do not assume sun.nio.ch.DirectBuffer

Addresses https://github.com/openmicroscopy/openmicroscopy/pull/1865#discussion_r8370560 by not assuming that `dispose()`'s,
```java
((sun.nio.ch.DirectBuffer) this.data).cleaner().clean();
```
is even possible with the loadable classes.

To see this in action, in `etc/omero.properties` set `omero.pixeldata.dispose=true` and do,
```
./build.py -f components/tools/OmeroJava/build.xml test -DTEST=integration/RawPixelsStoreTest
```
--no-rebase, following #1865 and #1884.