Fix the logic for Animated WebP images which contains a subsequence blend from the first frame, impact the SDAnimatedImage

[dreampiggy]

PR description

There are a critical bug in the SDAnimatedImageCoder protocol implementation for animatedImageFrameAtIndex:.

The implementation which use the optimization to avoid random access for webp frame, have a bug to detect which frame can be safelly blend start with.

The relative code can be checked there in YYImage-YYImageCoder.m

Bugfix

For example, this WebP image:
http://i1.chuimg.com/d76f3bed2a67462994cf0bac727f68b7_1080w_606h.webp

It has the following frame info:

Canvas size: 1080 x 606
Features present: animation transparency
Background color : 0xFFFFFFFF  Loop Count : 0
Number of frames: 24
No.: width height alpha x_offset y_offset duration   dispose blend image_size  compression
  1:  1080   606    no        0        0      125       none    no      49642       lossy
  2:  1080   606   yes        0        0      125       none   yes      49030       lossy
  3:  1080   606   yes        0        0      125       none   yes      50154       lossy
  4:  1080   606   yes        0        0      125       none   yes      50704       lossy
  5:  1080   606   yes        0        0      125       none   yes      48484       lossy
  6:  1080   606   yes        0        0      125       none   yes      49138       lossy

The bug screenshot:

The frame 2, which use blend == yes, should keep the frame 1's draw context, and blend on it. But however, current implementation ignore this.

See the debug result for the frame 2 representation, which does not have frame 1's blend draw at all. Instead it use a clear context for drawing (CGContextClearRect)

Fix

The correct result:

[kinarobin]

Thanks @dreampiggy. Now It works well in my project.