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

[syntheticmagus]

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.