Clear replay renderer when all instances are deleted.

[0mdc]

Motivation and Context

The batch renderer currently cannot delete individual instances. Because of this, it quickly reaches internal limits when repeatedly changing scenes. The batch renderer can however delete all instances at once.

This changeset clears the frame when the player detects that the applied keyframe is deleting all instances.

This implementation is preferred to an explicit clearFrame keyframe flag as it doesn't force the implementer (blender script, etc) to delete its instances in a specific way. However, it cannot detect leaking instances from a misused recorder.

How Has This Been Tested

Tested in python viewer by repeatedly changing scenes.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[eundersander]

Left a minor comment but otherwise LGTM!