Zeiss CZI: fix dimension calculations when expected planes are missing

See #12556.
Zeiss CZI: make sure pixel buffer is cleared

This prevents blank planes from containing garbage data.
Zeiss CZI: flag all files as being pixels files

This prevents pixel data from being accidentally included on the
companion file list.
Merge pull request #1386 from melissalinkert/12556

Zeiss CZI: fix dimension calculations when expected planes are missing