Add viewer for layout trace files.
Try to parse command line argument as file names

… when parsing as an absolute URL (without a base) fails.

Previously, arguments were parsed as URLs with the current working directory
converted to an URL and used as the base URL. This mostly worked for
relative filenames, except that `?` and `#` had a special meaning
and needed to be percent-encoded.

Fix #3340.
Give explicit file: URLs for reftests. Allow specifying a fragment id

The previous commit made #top be considered part of the file name in
../html/acid2.html#top
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
Merge pull request #3350 from glennw/layout-viewer

Add viewer for layout trace files.
Merge pull request #3365 from SimonSapin/command-line-argument-filenames

Try to parse command line argument as file names