Core: View synchronization

[klokan]

Correct implementation of synchronization of ol.View and Cesium.Camera:

• map center is the target of view
• resolution is calculated from the distance between the target and camera (resolution of the tile displayed in the center of screen)

Problems:

• camera has to be always directed towards the planet - what center should be set if this does not happen?
• Warning: on all the interactions once the terrain is turned on we depend on implementation and merge of this pull request: Camera-Terrain interaction CesiumGS/cesium#1817
  It is expected that camera never flies under the terrain.

Performance issues:
It is expensive to calculate the target and resolution too often (picking). We may need throttling and additional optimisations.

Note: When camera is tilted and resolution is calculated on target, the resolution can “jump” in ol.View - looking at a mountain, shift aside, looking far in a valley.

Note: With terrain turned on, the “resolution” of the view may differ substantially from altitude (different tiles with zoom level 16 on Everest and in Sahara at the same altitude).

When working on #2 (animated switch between 2D and 3D) the elevation value may be required to the fly from/to a perpendicular view on the target (original map center).

Warning: If somebody changes center in ol.View we will set perpendicular (reset) view at that position in Cesium in this first implementation. To preserve tilt, roll, etc. the ol3cesium method .setTarget() should be used instead.

[klokan]

Implemented, merged via #16 after discussion and review.

There is no need for reset of camera anymore in the existing implementation after ol.View changes Camera target - method is now called 'setCenter' (comment on the last Warning above)

Performance of synchronization may be improved with No rendering if invisible & lazy sync #19

[klokan]

A note: in Google Maps WebGL Globe the look towards the sky is not allowed - Google's answer to mentioned problem above. Try Shift drag in:
https://www.google.com/maps/@44.1798527,7.3205526,87149a,20y,8.57h,68.69t/data=!3m1!1e3