Support chunk for print media

[fsteimke]

I admit that this seems to be a rather esoteric configuration, so let me first explain the general idea and use case:

1. I'd like to support (at least partially) the copying of media files (see issue Feature request: a map with information necessary for copying media files #482 )
2. I have made an initial implementation, which executes copy instructions when the function file:copy is available (which is the case, by the way, for users of Oxygen). See PR 483.
3. Directly executable copy instructions are only possible, if both the source and the destination are known as absolute file paths. That means that i have to resolve every relative URI in ' img/@src` against the base-uri() of the HTML file.
4. In case of a regular, non-chunked transformation i don't know the destination of the transformations primary result (the HTML file), so i don't know the base-uri for copy instructions.
5. But chunked output can't be done without knowing the base-uri for HTML files, that's the rationale for $chunk-output-base-uri.
6. So i am able to generate executable copy instructions only when doing chunked output.
7. That is fine for online publishing, but what can i do if i am interested in PDF output?
8. Solution: Doing "fake chunking" by setting the parameter $chunk-include to the empty list. I will get only one file at the file path that is the combination of $chunk-output-base-uri and $chunk. And i get executable copy instructions since the base-uri in known.

This works well when i use docbook.xsl for transformation. It does not, however, when i use print.xsl for paged media. The Error message is

Error at char 56 in expression in xsl:variable/@select on line 437 column 45 of chunk-cleanup.xsl:
  FORG0002  Base URI {} is not an absolute URI. Found while atomizing the value of parameter
  $adjusted-uri in {supplied} on line 647. Found while atomizing the value of variable
  $adjusted-uri in {xsl:apply-templates} on line 621

This error is independent of the value of the parameter $chunk-include, it also happens with the default value. I think the reason is that print.xsl has never been tested with the chunk mechanism.

I don't know how to use the XSpec based test mechanism for this special case, so i made a test case in the attached archive. The included shell script should explain how it is to be used.
testcase.zip

Greetings, Frank