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
Fixes the table_percentage_width_a.html reftest with incremental reflow turned on.
auto merge of #3715 : cgaebel/servo/fix-table-percentage-width, r=pcwalton

This also enables incremental reflow by default. \o/

r? @pcwalton