PNG Plugin #267 Valgrind Warnings Uninitialized

Some output in non-live visualization (like png, density) did depend on uninitialized values (mainly `DataHeader::byte`).

Running tests right now to validate the valgrind output again.

lwfa with
- density output (-> not fixed, second uninitialized var in `DensityToBinary::operator()` back to #267)
- [x] png output (visualization)