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
Dump the flow tree with println, instead of error