return a Buffer from an export instead of writing to file
Hi guys, 

Is there a way (such an API) to acquire the thumbnail view's data from the layer inspector, which is highlighted in the below picture with a red rectangle?   I need this kind of data ( as a NSImage type) to do some image comparisons. 

![default](https://user-images.githubusercontent.com/1228472/39345492-46fb46f0-4a1b-11e8-8f5b-6e6aea73dffd.png)

Thanks in advance !
