Move dispatchMainThreadTasks out of updateView

[kring]

Tileset::updateView calls AsyncSystem::dispatchMainThreadTasks near its beginning. This is convenient, because cesium-native's clients don't need to remember to call this explicitly themselves. But it has some downsides:

One minor downside is that dispatchMainThreadTasks is called multiple times per frame, once per tileset, which has a small amount of overhead.

The bigger downside is that because a Tileset is actively running (we're executing one of its methods) when the main thread tasks are dispatched, we have to be very careful about what the tasks do, especially to the tileset. For example, if a main thread task (for example, one doing some work on behalf of a user interface) happens to destroy the Tileset, we're sure to crash or corrupt memory. That's what happened in CesiumGS/cesium-unreal#1047.

So, a better pattern is require users to call dispatchMainThreadTasks explicitly. This is only a small additional burden for users, and eliminates an entire class of bugs.

Originally mentioned here:
CesiumGS/cesium-unreal#1047 (comment)