voxel-model texture edits record into the build scope

voxel-model's texture canvas records into the build scope of the model's history. Painting a texture and shaping the model are one task in the build tab, so one undo stack walks back through both in order. The canvas waits for the workspace before it records.

Considered Options

• The active tab's scope. The same canvas would record into different stacks depending on the tab, and a stroke could be undone from a tab that does not show it.
• A paint scope. A third stack for the same tab, and the order between texture and model edits is lost.