Add more flexible target size options to ImageRequest.ThumbnailOptions

[kean]

In the previous version, the only available option was maxPixelSize:

// Before
ImageRequest.ThumbnailOptions(maxPixelSize: 64)

The main issue with maxPixelSize is that it creates a thumbnail that fits the target size. For example, if you have a wide image with 2000x400 pixel size and resize it to maxPixelSize: 40, the thumbnail will have the size 40x8 pixels. There was no way to create a 200x40 thumbnail, which is what you typically want to make sure the images have enough pixels to look good.

In addition to init(maxPixelSize:), there is now a new initializer with the parameters that directly match ImageProcessors.Resize, so it's easy to switch between them.

// After
ImageRequest.ThumbnailOptions(size: CGSize(width: 64, height: 64), contentMode: .aspectFill)

// Produces the same result as the following processor
ImageProcessors.Resize(size: CGSize(width: 64, height: 64), contentMode: .aspectFill)

The default contentMode is .aspectFill – the same as for ImageProcessors.Resize. And the default target size unit is .points.