[Request] Return autocrop() coordinates (cropped area X, Y, W and H)

**Is your feature request related to a problem? Please describe.**
I'm generating texture atlasses from other smaller images. I would like to crop the smaller images from surrounding transparency, which `.autocrop()` does fine, but I would need the X and Y coordinates of the cropped image relative to its original size.

**Describe the solution you'd like**
Since `.autocrop()` returns image instance, it can't return the coordinates, so maybe cropped instances could have `.cropX, .cropY, .cropWidth, .cropHeight` properties? Anything that would give me the crop X and Y coords will be fine (without scanning the images again).

Alternatively, there could be a new method, maybe named `boundingBox()` or something. This new method would return crop coordinates and size without cropping the image. That method would need to do the same calculations and accept the same arguments as `.autocrop()`. The result of the new `.boundingBox()` call could be passed to the `.crop(x, y, w, h)` method to do actual cropping.

**Describe alternatives you've considered**
I don't see a way with current API to get the crop coords.

Start by reading the existing autocrop() and crop(x, y, w, h) APIs to understand their current arguments and return behavior. Choose an API that exposes the crop coordinates without rescanning the image and preserves the existing cropping workflow; done means callers can obtain X, Y, width, and height and use them for cropping.

Written by the indexing model from the issue text.