[shapes] new shape-outside keyword for image data

Hi,

One of the most common cases for shape-outside is to shape an <img>
based on the alpha channel of its image. To accomplish this, authors do
the following:

<img src=foo.png alt="Something meaningful">

img {
  shape-outside: attr(src url);
}

This construction is pretty awkward. It seems to me like we've optimized
for the case where the <img>'s image is different from the image to be
used for shaping. We should have a keyword that captures the more common
case:

<img src=foo.png alt="Something meaningful">

img {
  shape-outside: bikeshed;
}

Another advantage of using a new keyword here is that not all images
have a corresponding URL that we can reference. For example:

<canvas></canvas>

canvas {
  shape-outside: bikeshed;
}


Ted