how to resize a bitmap on iOS

If you're not wed to CGBitmapContextRef, here are some UIKit-based simple image resizing routines. This extension offers resizes images by either cropping, scaling, aspect filling, and aspect fitting (analogous to a few of the main content modes that UIImageView offers).
//
//  UIImage+Resize.swift
//
//  Image resizing extension
//
//  Created by Robert Ryan on 19-May-11.
//  Ported to Swift by Robert Ryan on 12-Feb-15.
//  Modified for Swift 2 by Robert Ryan on 14-Oct-15
//  Modified for Swift 3 by Robert Ryan on 26-May-17
//  Modified for Swift 4 by Robert Ryan on 15-Feb-19
//
//  Inspired by http://ofcodeandmen.poltras.com/2008/10/30/undocumented-uiimage-resizing/
//  but adjusted to support AspectFill and AspectFit modes.
//
//  Copyright (c) 2015 Robert M. Ryan. All rights reserved.
//
//  This work by Robert M. Ryan is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
//  http://creativecommons.org/licenses/by-sa/4.0/

import UIKit

extension UIImage {

    /// Resize the image to be the required size, stretching it as needed.
    ///
    /// - parameter newSize:      The new size of the image.
    /// - parameter contentMode:  The `UIView.ContentMode` to be applied when resizing image.
    ///                           Either `.scaleToFill`, `.scaleAspectFill`, or `.scaleAspectFit`.
    ///
    /// - returns:                Return `UIImage` of resized image.

    func scaled(to newSize: CGSize, contentMode: UIView.ContentMode = .scaleToFill) -> UIImage? {
        switch contentMode {
        case .scaleToFill:
            return filled(to: newSize)

        case .scaleAspectFill, .scaleAspectFit:
            let horizontalRatio = size.width  / newSize.width
            let verticalRatio   = size.height / newSize.height

            let ratio: CGFloat!
            if contentMode == .scaleAspectFill {
                ratio = min(horizontalRatio, verticalRatio)
            } else {
                ratio = max(horizontalRatio, verticalRatio)
            }

            let sizeForAspectScale = CGSize(width: size.width / ratio, height: size.height / ratio)
            let image = filled(to: sizeForAspectScale)
            let doesAspectFitNeedCropping = contentMode == .scaleAspectFit && (newSize.width > sizeForAspectScale.width || newSize.height > sizeForAspectScale.height)
            if contentMode == .scaleAspectFill || doesAspectFitNeedCropping {
                let subRect = CGRect(
                    x: floor((sizeForAspectScale.width - newSize.width) / 2.0),
                    y: floor((sizeForAspectScale.height - newSize.height) / 2.0),
                    width: newSize.width,
                    height: newSize.height)
                return image?.cropped(to: subRect)
            }
            return image

        default:
            return nil
        }
    }

    /// Resize the image to be the required size, stretching it as needed.
    ///
    /// - parameter newSize:   The new size of the image.
    ///
    /// - returns:             Resized `UIImage` of resized image.

    func filled(to newSize: CGSize) -> UIImage? {
        let format = UIGraphicsImageRendererFormat()
        format.opaque = false
        format.scale = scale

        return UIGraphicsImageRenderer(size: newSize, format: format).image { _ in
            draw(in: CGRect(origin: .zero, size: newSize))
        }
    }

    /// Crop the image to be the required size.
    ///
    /// - parameter bounds:    The bounds to which the new image should be cropped.
    ///
    /// - returns:             Cropped `UIImage`.

    func cropped(to bounds: CGRect) -> UIImage? {
        // if bounds is entirely within image, do simple CGImage `cropping` ...

        if CGRect(origin: .zero, size: size).contains(bounds), imageOrientation == .up, let cgImage = cgImage {
            return cgImage.cropping(to: bounds * scale).flatMap {
                UIImage(cgImage: $0, scale: scale, orientation: imageOrientation)
            }
        }

        // ... otherwise, manually render whole image, only drawing what we need

        let format = UIGraphicsImageRendererFormat()
        format.opaque = false
        format.scale = scale

        return UIGraphicsImageRenderer(size: bounds.size, format: format).image { _ in
            let origin = CGPoint(x: -bounds.minX, y: -bounds.minY)
            draw(in: CGRect(origin: origin, size: size))
        }
    }

    /// Resize the image to fill the rectange of the specified size, preserving the aspect ratio, trimming if needed.
    ///
    /// - parameter newSize:   The new size of the image.
    ///
    /// - returns:             Return `UIImage` of resized image.

    func scaledAspectFill(to newSize: CGSize) -> UIImage? {
        return scaled(to: newSize, contentMode: .scaleAspectFill)
    }

    /// Resize the image to fit within the required size, preserving the aspect ratio, with no trimming taking place.
    ///
    /// - parameter newSize:   The new size of the image.
    ///
    /// - returns:             Return `UIImage` of resized image.

    func scaledAspectFit(to newSize: CGSize) -> UIImage? {
        return scaled(to: newSize, contentMode: .scaleAspectFit)
    }

}

extension CGSize {
    static func * (lhs: CGSize, rhs: CGFloat) -> CGSize {
        return CGSize(width: lhs.width * rhs, height: lhs.height * rhs)
    }
}

extension CGPoint {
    static func * (lhs: CGPoint, rhs: CGFloat) -> CGPoint {
        return CGPoint(x: lhs.x * rhs, y: lhs.y * rhs)
    }
}

extension CGRect {
    static func * (lhs: CGRect, rhs: CGFloat) -> CGRect {
        return CGRect(origin: lhs.origin * rhs, size: lhs.size * rhs)
    }
}
For Swift 2 rendition, see previous revision of this answer.
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