Once preview mode support for ImageReader and jimage is present, and once the jimage writer code can correctly create the new jimage with preview resources, new unit tests for both ImageReader and jimage should be added.

Currently, the plan for this work is to submit in parts, such that generating the jimage with preview resources comes after including the code to support it. This makes it impossible to submit the unit test changes for jimage.cpp and ImageReader.java as they are submitted.

These tests could go into the image writer changes, but I think it's better if they go in as a more focused change immediately after that's in.