Paint region based columns correctly in non-default writing-modes.

The column translation offset was calculated incorrectly.
The dirty rect intersection check was also wrong.

Added some documentation, to make it clear what's going on.

This patch is ported from WebKit.
https://bugs.webkit.org/show_bug.cgi?id=118506
Reviewed in WebKit by David Hyatt.

BUG=269417

Review URL: https://chromiumcodereview.appspot.com/22576002

git-svn-id: svn://svn.chromium.org/blink/trunk@158433 bbb929c8-8fbe-4397-9dbb-9b2b20218538