Changes

If `APNG_RENDER_OP_SKIP_FRAME` is present, then the decoder should not render the current frame as part of the animation. Though this flag can be set on any frame and must be honored, it is most useful for frame 0, to prevent the frame that would be visible to PNG viewers not supporting animation from being part of the animated frame set. If animation in the viewer is not desired or explicitly disabled by the user, the viewer should display frame 0 even if `SKIP_FRAME` is set on frame 0. This provides content authors with a means to provide a still image to be used in lieu of the full animation.

The `aDAT` chunk contains one frame for the animation. Any `aDAT` chunks must follow any `IDAT` chunks. All `aDAT` chunks must be adjacent in the PNG stream. Should a decoder receive an APNG stream with missing or out of order `aDAT` chunks, it is under no obligation to attempt to reorder the chunks and may treat that case as an error condition.