Fix perspective-workspace errors

[texodus]

• Fixes <perspective-viewer>'s delete() method to be async, and internally acquire the draw lock. This prevents a mid-executing call to draw() from losing access to the data due to an interrupting delete(), causing a segfault. This may happen when, e.g., a panel-close occurs in <perspective-workspace> quickly after a draw() was triggered on that panel, before it completes.
• Fixes threshold for light DOM update trigger, which caused errors when customizations are applied to a <perspective-viewer> in a workspace, which is subsequently closed.