The ImageWriter class contains a 'writePartial' method that allows an image
to be appended to a stream. Currently there is no way to set stream metadata
on such a sequence, or to declare the end of the sequence. New methods
'prepareWriteSequence' and 'endWriteSequence' should be added, and the
existing 'canWritePartial' and 'writePartial' methods should be renamed
to 'canWriteSequence' and 'writeToSequence'.