Change the inline_border test to use a PNG and regenerate baseline to work on OSX
auto merge of #1573 : larsbergstrom/servo/inline_border_reftest, r=larsbergstrom,metajack

This change at least ensures that it works on OSX and we can use that to avoid regressing the baseline. Will provide a separate PR that attempts to turn on content and ref tests on Linux to see how they are working on the buildbots right now.

r? @metajack