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

test/gfx/rgbgfx_test.cpp

@@ -2,10 +2,10 @@
 
 #include <algorithm>
 #include <array>
-#include <cassert>
-#include <cinttypes>
+#include <assert.h>
 #include <fcntl.h>
 #include <fstream>
+#include <inttypes.h>
 #include <limits>
 #include <png.h>
 #include <stdarg.h>