Subpixel rendering: Transforms on non-compositing layers leave bits behind when the...

Subpixel rendering: Transforms on non-compositing layers leave bits behind when the box boundaries changes.
https://bugs.webkit.org/show_bug.cgi?id=135786

Reviewed by Simon Fraser.

Since we don't control the transformed context painting's snapping strategy, our best bet to fully
include the transformed box when requesting for repaint is to use enclosing.
Pixelsnapping doesn't work here for all cases. For example, a scaled box at 0.8px can be anti-aliased painted
both at 0.5px and 1px, while with pixelsnapping we'd expect it to be painted starting from 1px;

Source/WebCore:

Test: fast/repaint/hidpi-transform-on-subpixel-repaintrect.html

* rendering/RenderBox.cpp:
(WebCore::RenderBox::computeRectForRepaint): enclose the result of the transform.

LayoutTests:

* fast/repaint/hidpi-transform-on-subpixel-repaintrect-expected.txt: Added.
* fast/repaint/hidpi-transform-on-subpixel-repaintrect.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@172373 268f45cc-cd09-0410-ab3c-d52691b4dbfc