Add Aperio 33003 test case
Prefix OpenJPEG error messages

OpenJPEG can produce obscure error messages:

    http://lists.andrew.cmu.edu/pipermail/openslide-users/2012-December/000546.html

so make it clear where they come from.
misc: Add program to scan for undecodable Aperio JP2K tiles
aperio: Don't choke on zero-byte JP2K tiles

Toby Cornish <tcornis3@jhmi.edu> discovered an Aperio 33005 slide in
which every tile in the right-most column of levels > 0 is zero bytes
long.  In this case, we were passing a NULL buffer and 0 length to
opj_cio_open(), causing it to configure the stream for writing rather
than reading.  The J2K decoder then tried to read the heap garbage in
the output buffer and failed with an error such as:

    00000000: expected a marker instead of 4880

Upon encountering such a tile, write out transparent pixels and continue.
hamamatsu: Check for empty MapFile key

Mathieu Malaterre <mathieu.malaterre@gmail.com> reported a VMU slide
with an empty MapFile key:

    http://lists.andrew.cmu.edu/pipermail/openslide-users/2012-December/000540.html

Explicitly test for this case and produce a sane error message.
hamamatsu: Support empty MacroImage value

Mathieu Malaterre reported a VMU slide with an empty MacroImage value,
which fails to open with:

    Could not read macro image: Empty input file

Don't choke on this case.
hamamatsu: Fix use-after-free in VMU error path
test/try_open: Add some helper functions
test: Allow test cases to read regions and check properties
test: Check quickhash-1 for each slide

to ensure the hashes are not inadvertently changed.