gfx: Binary search for the start position when iterating over the slices
in a text run.

This makes painting of text display items faster and matches what Gecko
does.
auto merge of #2621 : pcwalton/servo/binary-search-fix, r=brson

in a text run.

This makes painting of text display items faster and matches what Gecko
does.

Incorrect automatic derivation of `TotalOrd` broke it last time.

r? @brson