Change the inline_border test to use a PNG and regenerate baseline to work on OSX
Give up and set the initial size to be the same as the default size for reftests
Use synchronous image loading for ref test baseline writing
Update to rust-png with fix for truncation
auto merge of #1573 : larsbergstrom/servo/inline_border_reftest, r=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