Calling chart.regions([...]) does not update chart

Quick consecutive calls to `chart.regions(newRegions)` do not update the chart. Even calls spaced by 1 second do not work as expected.

In this example (https://jsfiddle.net/La5mfg67/) we do the following sequence:
- Set to `region1`. ✅
- Set to `[]`. ❌
- Set to `region2`. ❌
- Wait for 1s and set to `region3`. ❌
- Wait for 1s and set to `[]`. ✅
- Wait for 1s and set to `region4`. ✅
- Wait for 1s and set to `region5`. ❌

I was expecting:
- 0-1s: `region2` (with maybe some flashing?)
- 1-2s: `region3`
- 2-3s: `no regions`
- 3-4s: `region4`
- 4-5s: `region5`

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* **C3 version**: 0.7.15
* **D3 version**: 5.16.0
* **Browser**: Chrome 83.0.4103.61
* **OS**: MacOS Mojave 0.14.5