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.