Non-indexed images can still have a PLTE chunk. We should use
`sortRgb` instead of `sortIndexed` for them, since their pixels' colors
may not all be present in the PLTE chunk and would be unsortable.
* Use clang-tidy `misc-include-cleaner` for IWYU `#include` cleanup
* Use `std::optional<size_t>` instead of `ssize_t`
* Rename some functions in linkdefs.hpp
* Fix header order