低端机压缩5兆的图片必崩

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## Current use version？
#### 当前使用的版本是多少？

```
demo 最新版
```

## Will this problem occur in demo?
#### Demo能否复现这问题？

```
能
```

## Describe the problem or provide an error log？
#### 描述问题或提供错误log？

```
低端手机，压缩一个5mb大小的长图，必崩溃
java.lang.OutOfMemoryError: Failed to allocate a 208929252 byte allocation with 16777216 free bytes and 161MB until OOM
at dalvik.system.VMRuntime.newNonMovableArray(Native Method)
at android.graphics.BitmapFactory.nativeDecodeStream(Native Method)
at android.graphics.BitmapFactory.decodeStreamInternal(BitmapFactory.java:650)
at android.graphics.BitmapFactory.decodeStream(BitmapFactory.java:626)
at top.zibin.luban.Engine.compress(Engine.java:73)
at top.zibin.luban.Luban.compressReal(Luban.java:213)
at top.zibin.luban.Luban.compress(Luban.java:194)
at top.zibin.luban.Luban.access$1000(Luban.java:23)
at top.zibin.luban.Luban$1.run(Luban.java:146)
at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:243)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1133)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:607)
at java.lang.Thread.run(Thread.java:760)
```

Reproduce the crash in the demo with a 5 MB tall image on a low-end Android device, then start at Engine.java:73 and follow the compression call chain through Luban.java:213 and Luban.java:194. Done means compression completes without the reported OutOfMemoryError under the described conditions.

Written by the indexing model from the issue text.