Upgrade CESIUM_RTC to RTC_CENTER for b3dm

[lilleyse]

Any glTFs that contain the CESIUM_RTC extension can instead us the RTC_CENTER feature table property: CesiumGS/3d-tiles#263.

[javagl]

If I understood this correctly, then the setup was that:

• There was a GLB with CESIUM_RTC
• That GLB should be wrapped into a B3DM
• In that process, the CESIUM_RTC extension should be removed from the GLB, and added as the RTC_CENTER to the GLB

(I think there is even some (now:) "legacy" code for that, but I haven't analyzed that in all detail).

If this is correct, then... we'd now rather have to do the opposite: In the context of migrating from B3DM to GLB as the primary tile content, any RTC_CENTER could be converted into the CESIUM_RTC extension... but.... I think that even the CESIUM_RTC extension is somewhat outdated. (In fact, there are already some first steps for replacing the extension with a translation in a new root node, but this has to be tested/integrated more thoroughly as part of the migration functionality...)

So I think that this issue can be closed, because it is a mix of "obsolete" and "tracked elsewhere"....