Consume lots of memory when exporting

[Chao Cheng]

Describe the solution you'd like

RT

Should export and recycle image at the same time.

[Chao Cheng]

Improve a little when add isSynchronized to true of PHImageOptions.

But memory will still reach 1.3GB when export 50 images. But the export process is very fast and can be done successfully.

[Chao Cheng]

Fix in following two ways.

1. Add autoreleasepool to release image as soon as possible. link
2. When loading image from Photo, specify the minimuns size of all images.