The overhead from spawning a new ImageDecoder for every decoding job is
way too large and causing other problems as well (#5421)
Let's keep the same decoder open and reuse it as long as it's working.
If the ImageDecoder service crashes while decoding an image for us,
we now recover gracefully and simply return null.
This shields the browser from bugs in our image decoders.
The PIDs were used for sharing shbufs between processes, but now that
we have migrated to file descriptor passing, we no longer need to know
the PID of the other side.
The ImageDecoder service now returns a list of image frames, each with
a duration value.
The code for in-process image decoding is removed from LibWeb, an all
image decode requests are sent out-of-process to ImageDecoder. :^)
This won't scale super well to very long and/or large animations, but
we can work on improving that separately. The main goal here is simply
to stop doing any image decoding inside LibWeb.
Fixes#5165.
