If we have a valid PNG header with geometry info etc, we should still
display it as *something*, even if the image data itself is missing or
corrupted.
This matches the behavior of other browsers, and is something that
Cloudflare Turnstile checks for.
To achieve this, we split the PNG decoder's initialization into two
steps: "everything except reading frame data" and "reading frame data".
If the latter step fails, we yield a transparent bitmap with the
geometry from the PNG's IHDR chunk.
Before, libpng would use its own internal logging mechanism to print
non-fatal errors and warnings to stdout/stderr. This made it confusing
when trying to search the Ladybird codebase for those messages as they
didn't exist.
This commit uses `png_set_error_fn` from libpng to redirect those
messages to our own custom logging functions instead.
