Currently, if a SaveLayer contains no Entities, we always skip rendering it because it has no coverage. However, this is incorrect in cases where the entire subpass will be modified:

• The subpass blend mode is destructive.
• The subpass has a destructive color filter. (edit moved to issue [Impeller] Empty subpasses should not be elided when they have "destructive" color filters #137090)
• The subpass has a clear color set on it.

For all of these cases, the subpass also needs to expand to fill the current clip.