[css-borders] manipulating `border-image-source` before slice

https://drafts.csswg.org/css-backgrounds/#border-images

The border-image spec is missing the ability to manipulate border-image-source in the same ways we can manipulate background-image and mask-image. Note that this is about manipulating the border-image-source itself before it is sliced.

suggesting adding:

• border-image-source-size
• border-image-source-repeat
• border-image-source-position?
• etc
• comma separated multiple border-image-sources, -repeats, -sizes

use cases:

• Creating this sort of image to use for a corners only border should be doable with conic-gradient but currently isn't.

border-image-source: conic-gradient(#0000 0 270deg, #000 270deg);
border-image-source-size: 66.6% 66.6%;
border-image-source-repeat: repeat;

• other css pattern borders like checkerboard borders, diamonds etc... Many of which would currently require multiple images

• border-image-source-repeat would allow the use of a single image to be used as stamps around the border. One icon image repeated to create a 3x3 grid of that icon to use as a border-image-source.

border-image-source-size: 33.3% 33.3%;
border-image-source-repeat: repeat;

Start by reading the linked CSS Backgrounds border-images section and compare how background-image and mask-image are manipulated. Review the proposed source size, repeat, position, and multiple-source cases, then determine the required specification changes and use cases. Done means the CSSWG has a resolved, implementable definition for the requested behavior.