Utilize multi-update to remove stopgap measures in canvas and nativeEngine.ts

Stopgap measures were introduced in canvas and nativeEngine.ts to address capability shortcomings before multi-update was available, in both cases related to command gathering.

- The canvas uses a dispatched command gather step to collect commands for submission.
- nativeEngine.ts has a "scope" concept on the command buffer to overcome `NativeEngine`'s inability to request all commands "at the last moment."

Both of these stopgaps (and any others that are missing) can now be replaced by a `postUpdate` mechanism utilizing multi-update to request command gathering at an appropriate time. This will be a fairly invasive change affecting both JS and native, but it will allow us to remove the stopgap measures entirely and establish the pattern for sequenced update actions going forward.