Clarify that createImageBitmap fails with an SVG blob

This change makes it explicit that creating an ImageBitmap from a Blob
only supports *bitmap* file formats. This behaviour is consistent with
creating an ImageBitmap from an img element, which explicitly does not
support vector graphics.

Note: Supporting vector graphics would require defining a sizing
algorithm to determine the scale of image, or to have an alternate API
that has additional size parameters. See feature request in #923.

PR: #921

source

@@ -90443,9 +90443,10 @@ interface <dfn>ImageBitmapFactories</dfn> {
      by the <code>Blob</code> object's <code data-x="dom-Blob-type">type</code> attribute) giving the
      official type.</p></li>
 
-     <li><p>If the image data is not in a supported file format (e.g. it's not actually an image at
-     all), or if the image data is corrupted in some fatal way such that the image dimensions cannot
-     be obtained, then reject the promise with null, and abort these steps.</p></li>
+     <li><p>If the image data is not in a supported bitmap image file format (e.g. it's a vector
+     graphic or it's not an image at all), or if the image data is corrupted in some fatal way such
+     that the image dimensions cannot be obtained, then reject the promise with null, and abort
+     these steps.</p></li>
 
      <li><p>Create a new <code>ImageBitmap</code> object.</p></li>