Don't crash on invalid utf-8 in the HTML parser.

This was regressed by the html5ever landing.
auto merge of #3712 : Ms2ger/servo/parser-crash, r=Manishearth

This was regressed by the html5ever landing.
Remove render backend option as it doesn't work and confuses people.
auto merge of #3711 : glennw/servo/cleanup-opts, r=pcwalton

r? @pcwalton @metajack
Use opts as a global, to avoid cloning and passing the struct all over the code.
auto merge of #3729 : glennw/servo/opts-cleanup, r=mbrubeck