Store previous_asset in PrepareNextFrameAssets.

[larsraph]

Issue

In prepare_assets the previous_asset is passed to A::prepare_asset however when an asset is enqueued into the PrepareNextFrameAssets it's previous asset is removed from RenderAssets and dropped. When consuming the queue is attempts to read the value from the RenderAssets which always returns None.

Solution

This change simply stores the previous_asset in the PrepareNextFrameAssets queue.

Alternative Solution

I find it dubious that the RenderAsset has to be removed from RenderAssets in the first place. The reason given is that "we remove previous here to ensure that if we are updating the asset then any users will not see the old asset after a new asset is extracted, even if the new asset is not yet ready or we are out of bytes to write." However, I would personally expect to keep stale data until the updated data arrives. If we do that then there would be no need to store the previous asset.

Testing

• I have not tested this... How would I go about testing this?

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