Add canvas and image support

Adds support for drawing "pixels" and displaying
images in the terminal.

src/Spectre.Console/Widgets/Canvas.cs

@@ -0,0 +1,151 @@
+using System;
+using System.Collections.Generic;
+using Spectre.Console.Rendering;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Represents a renderable canvas.
+    /// </summary>
+    public sealed class Canvas : Renderable
+    {
+        private readonly Color?[,] _pixels;
+
+        /// <summary>
+        /// Gets the width of the canvas.
+        /// </summary>
+        public int Width { get; }
+
+        /// <summary>
+        /// Gets the height of the canvas.
+        /// </summary>
+        public int Height { get; }
+
+        /// <summary>
+        /// Gets or sets the render width of the canvas.
+        /// </summary>
+        public int? MaxWidth { get; set; }
+
+        /// <summary>
+        /// Gets or sets a value indicating whether or not
+        /// to scale the canvas when rendering.
+        /// </summary>
+        public bool Scale { get; set; } = true;
+
+        /// <summary>
+        /// Gets or sets the pixel width.
+        /// </summary>
+        public int PixelWidth { get; set; } = 2;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Canvas"/> class.
+        /// </summary>
+        /// <param name="width">The canvas width.</param>
+        /// <param name="height">The canvas height.</param>
+        public Canvas(int width, int height)
+        {
+            Width = width;
+            Height = height;
+
+            _pixels = new Color?[Width, Height];
+        }
+
+        /// <summary>
+        /// Sets a pixel with the specified color in the canvas at the specified location.
+        /// </summary>
+        /// <param name="x">The X coordinate for the pixel.</param>
+        /// <param name="y">The Y coordinate for the pixel.</param>
+        /// <param name="color">The pixel color.</param>
+        public void SetPixel(int x, int y, Color color)
+        {
+            _pixels[x, y] = color;
+        }
+
+        /// <inheritdoc/>
+        protected override Measurement Measure(RenderContext context, int maxWidth)
+        {
+            if (PixelWidth < 0)
+            {
+                throw new InvalidOperationException("Pixel width must be greater than zero.");
+            }
+
+            var width = MaxWidth ?? Width;
+
+            if (maxWidth < width * PixelWidth)
+            {
+                return new Measurement(maxWidth, maxWidth);
+            }
+
+            return new Measurement(width * PixelWidth, width * PixelWidth);
+        }
+
+        /// <inheritdoc/>
+        protected override IEnumerable<Segment> Render(RenderContext context, int maxWidth)
+        {
+            if (PixelWidth < 0)
+            {
+                throw new InvalidOperationException("Pixel width must be greater than zero.");
+            }
+
+            var pixels = _pixels;
+            var pixel = new string(' ', PixelWidth);
+            var width = Width;
+            var height = Height;
+
+            // Got a max width?
+            if (MaxWidth != null)
+            {
+                height = (int)(height * ((float)MaxWidth.Value) / Width);
+                width = MaxWidth.Value;
+            }
+
+            // Exceed the max width when we take pixel width into account?
+            if (width * PixelWidth > maxWidth)
+            {
+                height = (int)(height * (maxWidth / (float)(width * PixelWidth)));
+                width = maxWidth / PixelWidth;
+            }
+
+            // Need to rescale the pixel buffer?
+            if (Scale && (width != Width || height != Height))
+            {
+                pixels = ScaleDown(width, height);
+            }
+
+            for (var y = 0; y < height; y++)
+            {
+                for (var x = 0; x < width; x++)
+                {
+                    var color = pixels[x, y];
+                    if (color != null)
+                    {
+                        yield return new Segment(pixel, new Style(background: color));
+                    }
+                    else
+                    {
+                        yield return new Segment(pixel);
+                    }
+                }
+
+                yield return Segment.LineBreak;
+            }
+        }
+
+        private Color?[,] ScaleDown(int newWidth, int newHeight)
+        {
+            var buffer = new Color?[newWidth, newHeight];
+            var xRatio = ((Width << 16) / newWidth) + 1;
+            var yRatio = ((Height << 16) / newHeight) + 1;
+
+            for (var i = 0; i < newHeight; i++)
+            {
+                for (var j = 0; j < newWidth; j++)
+                {
+                    buffer[j, i] = _pixels[(j * xRatio) >> 16, (i * yRatio) >> 16];
+                }
+            }
+
+            return buffer;
+        }
+    }
+}