read CZI file that missing Scene dimension

While almost all CZI slide files contains SizeS in xml metadata and 'S'
dimension in dimension entry, an exception is the embedded SlidePreview
CZI file, which missing Scene dimension in both xml metadata and
dimension entry. The embedded SlidePreview is valid CZI as Zeiss ZEN
software can view it, when it is extracted and save as individual file.

The SlidePreview CZI is the only 48bits color image available, which
makes them good candidates for testing zstd1 mode hi low bytes pack.

Signed-off-by: Wei Chen <chenw1@uthscsa.edu>