Also fixes calculation of background-position percentages:
Rather than multiplying the container size by a percent and aligning the top
left of the image at the resulting width, we also need to subtract a
corresponding percent of the image size, per
http://dev.w3.org/csswg/css2/colors.html#propdef-background-position
"A value pair of '100% 100%' places the lower right corner of the image in the
lower right corner of the padding box. With a value pair of '14% 84%', the
point 14% across and 84% down the image is to be placed at the point 14%
across and 84% down the padding box."
This still needs a lot of work, but it covers the basic
cases and improves wikipedia while passing all existing tests.
Tweak reftest to deal with linux/travis black background.
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.
