Improper use of scrollLeft and scrollRight in resetTriggers in non-default writing modes

[fred-wang]

cc @cathiechen

The resetTriggers function sets the scrollLeft/scrollTop properties of the expand/contract children of the resize-triggers div to a non-negative value.

However, this is incorrect in some non-default writing modes where scrollLeft/scrollTop must be non-positive. See https://people.igalia.com/fwang/scrollable-elements-in-non-default-writing-modes/ ; Chrome is going to change to use the standard behavior https://www.chromestatus.com/feature/5759578031521792

I'm not really sure how this polyfill for ResizeObserver is working, but a possible fix would be to force "direction: ltr; writing-mode: horizontal-tb;" on the expand/contract div. Or check the actual CSS direction/writing-mode to determine whether to use positive/negative values.

[fred-wang]

@sdecima I opened #62 ; please take a look ;-)