Set billboard opacity

[gberaudo]

Handle opacity property in the converter.

[gberaudo]

This PR allows modifying the opacity of an ol3 image Point dynamically.
It is made possible following the recent ol3 PR merge openlayers/openlayers#3044.

[ahocevar]

Looks good. Please merge.

[schmidtk]

This works if the opacity is set at the style level, but it may be set at the layer level. Are there plans to handle that case?

[gberaudo]

Unfortunately, this OL3 mechanism does not have equivalent in Cesium.
A good solution would avoid destroying and recreating all primitives.
It would require Ol3 to propagate layer changes in a finer way.

[ahocevar]

I'll take a look. Maybe there is a way.

[ahocevar]

@schmidtk As far as I can tell, this change also works for an opacity set on the layer level. The style that is queried for the opacity still comes from olcs.core.computePlainStyle, which takes both feature and layer styles into account and executes the style function for each feature. Can you confirm?

[ahocevar]

I see now what you mean @schmidtk: an opacity set by ol.layer.Base#setOpacity. I'll see if I can come up with something.

[ahocevar]

@schmidtk I came up with #166 for layer opacity support.