Show a mask as soon as it is made

Choosing a category is asking what it selected, and for a subject or a
category that question had no other answer on screen: the model's outline is
not derivable from anything visible, a fresh layer carries no adjustment to
judge it by, and the list it was chosen from says "architecture 23%" without
saying which 23%. The control that draws the mask existed but had to be found
and pressed, a panel's height away from the list the choice was made in.

So a new layer arrives with its mask showing, from the resting position only.
Somebody who has chosen the alpha or the outline keeps it, and nothing re-arms
in the background — every caller is a press that asked for a new mask.

That makes the canvas depend on how a layer arrived, which is correct and
worth stating: a session that has just made a mask draws a frame that a
session which read the same mask out of a sidecar does not. Viewing state is
not edit state and does not travel in a file, and
`a_stored_mask_renders_exactly_what_the_model_rendered` now says so at both
ends.

ui/dr-ui/src/masks_ui.rs

@@ -1027,14 +1027,11 @@ pub(crate) fn wire(
             // until something is painted into it, so a press that made a row
             // and left the pointer in "select" would be indistinguishable from
             // a button that did nothing. The same reasoning as `on_part_added`
-            // below, and the tool's own handler turns the mask view on.
+            // below. Showing the mask is `DevelopSession::show_new_mask`'s
+            // job and every route to a new layer goes through it, so only the
+            // tool is set here.
             if made {
                 w.global::<Masking>().set_tool(1);
-                if let Some(s) = session.borrow_mut().as_mut() {
-                    if s.mask_view() == 0 {
-                        s.set_mask_view(1);
-                    }
-                }
             }
             sync(&w, &session);
             sync_rows(&w, &rows, &session);