Problem creating jpeg tiles when native JAI is not installed

Problem reported in http://jira.codehaus.org/browse/GEOS-4492

Assessed it only manifests if native JAI is not installed, seems to be a problem with the com.sun.imageio.plugins.jpeg.JPEGImageWriter not supporting cropped RenderedImages when MetaTile writes single tiles out of the meta tile image.

Note: this may be a regression.
