VideoPress: take advantage of the edit-when-uploading to store the video sizes


Maybe we can get the video size initially, even before to get uploaded. It would be nice to store the size in the block attribute to reduce the jumps when rendering the block.

Start by tracing the VideoPress block's edit-when-uploading flow and how its block attributes are used while rendering. Determine where the video size becomes available before upload completes and define the rendered behavior that should avoid layout jumps. Done means the size is stored in the block attribute early enough to reduce those jumps.

Written by the indexing model from the issue text.