Fix the definition of pixelated for createImageBitmap

This adopts the CSS definition from https://drafts.csswg.org/css-images-3/#valdef-image-rendering-pixelated, adapted for the HTML context.

Closes whatwg#3485.

source

@@ -115930,9 +115930,15 @@ dictionary <dfn dictionary>ImageBitmapOptions</dfn> {
   interpolation quality to use when scaling images.</p>
 
   <p>The "<dfn enum-value for="ResizeQuality"><code
-  data-x="dom-ResizeQuality-pixelated">pixelated</code></dfn>" value indicates a preference to scale
-  the image that maximizes the appearance. Scaling algorithms that "smooth" colors are acceptable,
-  such as bilinear interpolation.</p>
+  data-x="dom-ResizeQuality-pixelated">pixelated</code></dfn>" value indicates a preference for
+  scaling the image to preserve the pixelation of the original as much as possible, with minor
+  smoothing as necessary to avoid distorting the image when the target size is not a clean multiple
+  of the original.</p>
+
+  <p>To implement "<code data-x="dom-ResizeQuality-pixelated">pixelated</code>", for each axis
+  independently, first determine the integer multiple of its natural size that puts it closest to
+  the target size and is greater than zero. Scale it to this integer-multiple-size using nearest
+  neighbor, then scale it the rest of the way to the target size using bilinear interpolation.</p>
 
   <p>The "<dfn enum-value for="ResizeQuality"><code data-x="dom-ResizeQuality-low">low</code></dfn>"
   value indicates a preference for a low level of image interpolation quality. Low-quality image
@@ -115950,7 +115956,8 @@ dictionary <dfn dictionary>ImageBitmapOptions</dfn> {
   <p class="note">Bilinear scaling is an example of a relatively fast, lower-quality image-smoothing
   algorithm. Bicubic or Lanczos scaling are examples of image-scaling algorithms that produce
   higher-quality output. This specification does not mandate that specific interpolation algorithms
-  be used unless the value is "<span data-x="dom-ResizeQuality-pixelated">pixelated</span>".</p>
+  be used, except for "<code data-x="dom-ResizeQuality-pixelated">pixelated</code>" as described
+  above.</p>
 
   </div>