cpp: FormatWriter: Drop unused variable 'planes'
cpp: tiff: Add INDEXED tag
cpp: tiff: IFD provides PhotometricInterpretation accessors
cpp: CoreMetadata default falseColor to true

This is to match the Java implementation.
cpp: Correct CoreMetadata docstrings
cmake: TIFF and PNG libraries are required
Merge pull request #1374 from rleigh-dundee/cpp-fixes

C++: Collected small fixes