Build pipelines before the layers that need them

Reviewing the previous fix, which was itself unreviewed. `update` gave
each list the bind group layout of the pipeline that draws it by zipping
the layer's lists against `self.primitives`, but built those pipelines
afterwards -- so on any pass where one did not exist yet the zip yielded
nothing, and those lists kept no bind group and drew nothing. Measured
on startup: four layers had content while `self.primitives` was still
empty. It only looked right because those layers were marked dirty again
on a later frame and rebuilt then.

`build_pipelines` now runs before the layers, and the pairing is indexed
rather than zipped, so a primitive drawn before it was registered panics
instead of silently leaving its list unbuilt.

core/src/render/mod.rs

@@ -118,6 +118,10 @@ impl UiRenderNode {
         ui: &mut UiData,
         ui_render: &mut UiRenderState,
     ) {
+        // Before the layers: a list is given the bind group layout of the
+        // pipeline that will draw it, so every registered primitive needs one
+        // by the time a layer is reached.
+        self.build_pipelines(device, &ui.primitives);
         self.active.clear();
         for (i, draws) in ui_render.layers.iter_mut() {
             self.active.push(i);
@@ -139,13 +143,16 @@ impl UiRenderNode {
                 rlayer
                     .primitives
                     .resize_with(draws.primitives().len(), || ListBuffers::new(device));
-                for ((list, draws), pipeline) in rlayer
+                for (id, (list, draws)) in rlayer
                     .primitives
                     .iter_mut()
                     .zip(draws.primitives())
-                    .zip(&self.primitives)
+                    .enumerate()
                 {
-                    list.update(device, queue, &pipeline.data_layout, draws);
+                    // Indexed, not zipped: a missing pipeline should say so
+                    // rather than quietly leave the list unbuilt.
+                    let layout = &self.primitives[id].data_layout;
+                    list.update(device, queue, layout, draws);
                 }
                 rlayer
                     .textures
@@ -160,7 +167,6 @@ impl UiRenderNode {
                 draws.updated = false;
             }
         }
-        self.build_pipelines(device, &ui.primitives);
         if ui.masks.changed {
             ui.masks.changed = false;
             if self.masks.update(device, queue, &ui.masks[..]) {