[css-backgrounds-4] Background origin for x and y axes

It would be useful to have background-origin-x and background-origin-y properties, similarly to background-position-x and background-position-y.

Consider this example:

<input class="with-icon">

input
{
  padding: 20px;
}

/* old way */
.with-icon
{
  padding-left: 40px; /* icon will be placed within this left padding */
  background: no-repeat 14px 50%/auto 60% url('image.svg'); /* icon size has magic numbers */
}

/* new way */
.with-icon
{
  padding-left: 40px;
  background: no-repeat 14px 50%/auto contain url('image.svg'); /* icon size is relative to content area of the input */
  background-origin-y: content-box;
}

We can't use simply background-origin here, because the background position of the icon would we within the content boundaries of the input, and thus would require to have negative numbers.

Start by reviewing the existing background-position-x and background-position-y definitions, then compare their behavior with the background-origin proposal in this issue. Determine whether separate x and y origin properties fit the CSS Backgrounds specification; completion requires an agreed specification change and corresponding wording.