Revert "Fix pixel bleed in BorderImage"

Revert "Fix pixel bleed in BorderImage"

This reverts commit a9238292145e05d36be7c35bdd50829c6400a3de.

It also reverts follow-up commits:
  87755d0437413cfb875c50e3dfa53030601567c9
  c2c710e5a8d66fa696276aa69c2e3b00436eefe5
  cc8a76ce8ea76afe912902067e95ca2abf9e482f

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>