Open: the first frame uses the layouts the open made for type objects

Opening a document laid out every type object for its bounds and
baselines and threw the layouts away, and the first frame laid the same
objects out again. The open now keeps them, up to 128 MiB, and a frame
takes each one once.

A frame takes a layout only for a text object equal to the one laid out,
and only while the fonts, the system fallback setting and the hyphenation
exceptions are unchanged. When the last frame being drawn ends, the rest
are dropped if a frame took one, and all of them are dropped after eight
frames that take none. A layout with bitmap or color glyphs and a text
object with a gradient are not kept.

crates/engine/src/cmd/typecmd.rs

@@ -298,6 +298,18 @@ pub(crate) fn refresh_bounds(t: &mut TextObject) {
     t.cached_baselines = lay.baselines();
 }
 
+/// [`refresh_bounds`] for a document that is opening: the first frame takes the layout
+/// ([`vectorcraft_text::take_kept_layout`]) instead of laying the object out again.
+pub(crate) fn refresh_bounds_for_first_frame(t: &mut TextObject) {
+    t.normalize_paras();
+    auto_height(t);
+    let stamp = vectorcraft_text::LayoutStamp::now();
+    let lay = vectorcraft_text::layout(vectorcraft_text::FontDb::global(), t);
+    t.cached_bounds = Some(lay.bounds);
+    t.cached_baselines = lay.baselines();
+    vectorcraft_text::keep_layout(t, stamp, lay);
+}
+
 /// The text-space bounds of area type's frame, when Auto Size can fit it: a rectangle (one
 /// subpath, as large as its bounds) of horizontal type in one row, with some height.
 fn auto_height_frame(t: &TextObject) -> Option<vectorcraft_geom::Rect> {