Expose the AccessorView of PropertyAttributePropertyView

[j9liu]

Added in support of CesiumGS/cesium-unreal#1685.

In the case of voxels, we use a megatexture to store each renderable tile's data. If a property matches the type of a texture in Unreal (e.g. float properties), then it's more performant to copy the entire buffer at a time into the texture, instead of converting its values one-by-one.

To do that, we'd need access to the data pointer of the accessor used by the PropertyAttributePropertyView, hence the accessorView method.

Author checklist

• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• [ ] I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

[kring]

Thanks @j9liu!