问题：SDWebImage IOS 9 无法成功抓取png格式图片
```
an 13 01:05:54Dota2Heros[6498] : CGContextDrawImage: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Jan 13 01:05:54Dota2Heros[6498] : CGBitmapContextCreateImage: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
2016-01-13 01:05:54.656 Dota2Heros[6498:383059] http://cdn.dota2.com/apps/dota2/images/abilities/drow_ranger_marksmanship_hp1.png
Jan 13 01:05:54Dota2Heros[6498] : CGBitmapContextCreate: unsupported parameter combination: set CGBITMAP_CONTEXT_LOG_ERRORS environmental variable to see the details
Jan 13 01:05:54Dota2Heros[6498] : CGContextDrawImage: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Jan 13 01:05:54Dota2Heros[6498] : CGBitmapContextCreateImage: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
```
解决方法：iOS SDWebImage can not load PNG image · Issue #1335 · rs/SDWebImage · GitHub
ios学习中, 感谢 @mythodela!
