Fixed using info dictionary when writing multiple APNG frames - #5611

Two changes here.

1. Fixed using info dictionary when writing multiple APNG frames.

duration can come from encoderinfo or info when saving multiple frames.

The duration variable is only part of the individual frame's encoderinfo when it is a list or tuple though, or if it was part of the actual image's encoderinfo.

So when it comes to fetch it for writing, it is ignored.

The same applies to disposal and blend.

2. Pillow/src/PIL/PngImagePlugin.py

   Lines 1122 to 1125 in 696b82e

   	if "duration" in previous["encoderinfo"]:
   	previous["encoderinfo"]["duration"] += frame_duration
   	else:
   	previous["encoderinfo"]["duration"] = frame_duration

Duration is either specified for every frame (in which case the condition is true), or a single value is used (in which case previous["encoderinfo"]["duration"] = frame_duration is unnecessary). So the else code can be removed.