Highlight selected text in input fields

Fixes #9993.  This does not yet allow stylesheets to set the selection colors;
instead it uses a hard-coded orange background and white foreground.

components/layout/inline.rs

@@ -354,6 +354,7 @@ impl LineBreaker {
             let need_to_merge = match (&mut result.specific, &candidate.specific) {
                 (&mut SpecificFragmentInfo::ScannedText(ref mut result_info),
                  &SpecificFragmentInfo::ScannedText(ref candidate_info)) => {
+                    result_info.selected == candidate_info.selected &&
                     util::arc_ptr_eq(&result_info.run, &candidate_info.run) &&
                         inline_contexts_are_equal(&result.inline_context,
                                                   &candidate.inline_context)