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

include/gfx/png.hpp

@@ -3,8 +3,8 @@
 #ifndef RGBDS_GFX_PNG_HPP
 #define RGBDS_GFX_PNG_HPP
 
-#include <fstream>
 #include <stdint.h>
+#include <streambuf>
 #include <vector>
 
 #include "gfx/rgba.hpp"