Occasional redundant resize notifications

[tibineagu]

At this line in ResizeObserver.vue, the notify event is being emitted for every detected "resize" of the _resizeObject.

However, in some situations, if you debug the values of this.$el.offsetWidth and this.$el.offsetHeight, they are the same across all the occurrences of the resize event (which leads me to think that they're likely caused by sub-pixel varations in the height of the object).

This is related to this issue in vue-virtual-scroller where the redundant notifications generate an infinite loop of height recalculations in the DynamicScroller component.

I think the correct behaviour should be to only trigger the resize notification if the calculated dimensions of the wrapper element have also changed.

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EDIT: Opened PR #8 for this