Some refactoring and cleanup (#1806)

* 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

src/gfx/rgba.cpp

@@ -3,6 +3,7 @@
 #include "gfx/rgba.hpp"
 
 #include <algorithm>
+#include <array>
 #include <math.h>
 #include <stdint.h>