Sketch on an origin plane faces the side you clicked (#1587)

Fixes #1586.

crates/doc/src/format/pending/flipped-sketch-plane.rs

@@ -0,0 +1,5 @@
+//! A sketch can sit on a plane seen from its other side: `{"type": "flipped", "base": <plane>}`
+//! (with `"about_y": true` when it turns about the plane's y axis rather than its x axis). New
+//! sketches started from behind an origin or construction plane save this, so they face the
+//! viewer. Older designs have no flipped planes and open unchanged; the last release refuses a
+//! design that has one (it asks for an update) instead of putting the sketch on the wrong side.