Summary: For some reason, adding `border-style: solid` also adds a 3px
border around elements, which means that all of the rules that we
created are 3px too large. This sets the default size to 0 for all the
edges, which makes them correct.
Test plan:
- See that `a\rule{0em}{0em}b` produces no visible rule.
- See the new screenshots look reasonable.
The combination of jspngopt and pako should eliminate possible causes for
different PNG encodings, although the core reason for #325 remains unknown.
Pako has poorer compression rates than native libz, but optimization can
counter that effect, and actually reduce the size of the screenshots.
The screenshots for LimitControls and UnsupportedCmds on Firefox used to
exhibit subpixel rendering before, for reasons unknown. The regenerated
versions don't exhibit this. See #324 for a discussion.
