Preprocess: fixed keep ratio and changed split behavior

[Milly]

In Train > Preprocess images > Split oversized images to two,

Fixed bug

• If a non-square is specified, it will not keep the aspect ratio of the image.

This PR fixed it.

Changed split behavior

Old behavior,

• When an too long image is input, the center of the image disappears.
• Just split it in two.

New,

• Added Split image threshold.
  • The wider the cropped area or the higher the threshold, the more splitable.
• Added Split image overlap ratio.
  • A high overlap ratio splits the image into many pieces.

Sample

Specified:

• Width: 512
• Height: 768