Add scale to Cesium3DTileStyle

#### Proposal
Add a `.scale` property to the `Cesium3DTileStyle` which scales _Vector3DTilesPoint_ features.

#### Use Case
Points representing quantities (eg. geostatistical values) can be scaled accordingly, _even if they have an image icon_. Currently, only the `pointSize` can be set, which is ignored if there is an `image`.

#### Proposed Solution:
- Add a `scale` propety to `Cesium3DTilePointFeature`
- Add a `scale` property to `Cesium3DTileStyle`
- Evaluate `scale` in `Vector3DTilesPoints`

You can see the proposed changes in [this commit](https://github.com/virtualcitySYSTEMS/cesium/commit/32570f83d205678d1a7158d8276791487ca47bb2). I would gladly open up a PR for this with cesium and the 3d-tiles-next spec repo.

Start by reviewing the linked proposed commit, then inspect Cesium3DTilePointFeature, Cesium3DTileStyle, and Vector3DTilesPoints. Trace how pointSize and image are currently evaluated. Done means adding the scale properties and applying the style value to Vector3DTilesPoint features, including features with image icons.

Written by the indexing model from the issue text.