Partial fix for background images on wikipedia pages.

This fixes the visual artifacts seen at the top of wikipedia pages by:
 - Setting clipping rect to avoid images going outside their bounds.
 - Handling case of background-position being >= 100% such that wrapping is required.

However, the gradient is not currently visible on wikipedia. This relies on bug #1997 being fixed.
Fix reftest now that the background image code clips correctly.
Merge pull request #3283 from glennw/bg-image

Partial fix for background images on wikipedia pages.
Coalesce pending reflow messages so they don't queue up.
Merge pull request #3294 from glennw/guardian-layout

Coalesce pending reflow messages so they don't queue up.
layout: Don't crash on floated generated content.

It doesn't construct the float correctly, but at least it doesn't crash
anymore.

Fixes Reddit.

Closes #3287.