Revert "Fix pixel bleed in BorderImage"

This reverts commit a9238292.

It also reverts follow-up commits:
  87755d04
  c2c710e5
  cc8a76ce



The new implementation is much too slow because it will cause each
section of the border image to be uploaded to the atlas, and it will
not be shared between different border images nor between different
sizes of the border. The performance regression is so significant
that it's not worth it for the original bug fix.

We leave the tests around, so some of those might regress due to
this.

Change-Id: I3f56f4564255ebb77e6487881a6a11b19ad0234e
Task-number: QTBUG-42288
Task-number: QTBUG-35838
Reviewed-by: Michael Brasser <michael.brasser@live.com>