Implement background-position keyword values

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."
Merge pull request #3296 from mbrubeck/background-position

Support CSS2 background-position keyword and percent values. r=SimonSapin