the ImageShader class should update the VM with allocation information when an an image is actually set to it.

Once we can do that, we should also report the size through to Canvas/Picture when the shader is actually applied there.

See also: #58438, flutter/engine#18706

Blocked on: https://dart-review.googlesource.com/c/sdk/+/149245

/cc @rmacnak-google @chinmaygarde @liyuqian