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 #1623 : larsbergstrom/servo/inline_border_reftest, r=metajack

r? @metajack

These are all commits you've previously reviewed, but without the image_border change madness. We're seeing some intermittent reftest failures, so I'd like to separately land these.