In the previous fix, we were still drawing IDAT data to the reference
frame even when no fcTL was present. This would cause rendering issues
when subsequent frames use APNG_BLEND_OP_OVER blending mode, as they
would composite over the incorrect reference frame. This commit adds a
simple check to properly skip any frame without an fcTL chunk.
Before this change, IDAT data was mistakenly always included in the
animation. Now we only include frames with explicit fcTL chunks.
As per the PNG spec (third edition):
"The static image may be included as the first frame of the animation
by the presence of a single fcTL chunk before IDAT. Otherwise, the
static image is not part of the animation."
We also fall back to the IDAT data when APNG has acTL but no fcTL
chunks. Test image is 062.png from fDAT-inherits-cICP.html from WPT.
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.
