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 #1400 from melissalinkert/rebased/dev_5_0/12556

Zeiss CZI: fix dimension calculations when expected planes are missing (rebased onto dev_5_0)