When we image_new_from_source, the source object has some of the loader
state: it tracks the current read position of the load library. This
means that we mustn't keep source loaders in the operation cache, since
a second call could give a different result because the source object
read position might have changed.
Also: add a rewind to get_flags_source in spngload, and jp2k needs to
tag its load region as having no thread ownership or you'll get assert
fails in the test suite with debug enabled.
cb58d7d960