Draw textures separately, and keep them out of Primitives

Interleaving textures with primitives was solving a problem that does not
exist: within a layer, order is already undefined because freeing an
instance swap-removes it, and layering is what layers are for. So the run
batching is gone.

A layer is now `LayerDraws`: a `Primitives` and a texture `InstanceList`
side by side, with `updated` covering both. `Primitives` holds only
primitives again -- its instance list plus the group-1 data those
instances read -- and `InstanceList` is the shared push/free/apply_free
the two lists have in common rather than a second copy of it.
`PrimitiveHandle` names which list with `InstanceKind`, and
`PrimitiveChange` carries the same, since the two index independently.
The handle no longer carries a group-1 index at all: a primitive
instance already records where its entry is.

The renderer gives each layer a second instance buffer and draws its
textures one at a time after the instanced draw, each binding its own
group 2.

Review fixes alongside: `GlyphAtlas::allocate` returns the `PageUpload`
it reserved instead of a bare tuple; a page or image region uploads
through a new `write_region`, which passes the row stride to
`write_texture` rather than copying the rectangle out first.

Verified by replaying taps into the `tabs` example: three images added
and one deleted leaves two drawn with two live texture slots, and the
masked text-edit tab still clips, with the images freed on tab switch.

core/src/ui/render_state.rs

@@ -1,13 +1,13 @@
 use crate::{
-    ActiveData, Axis, IdLike, MaskIdx, Painter, PixelRegion, PrimitiveLayers, SizeCtx,
-    StrongWidget, UiRegion, UiRsc, UiVec2, WidgetId, Widgets,
+    ActiveData, Axis, DrawLayers, IdLike, MaskIdx, Painter, PixelRegion, SizeCtx, StrongWidget,
+    UiRegion, UiRsc, UiVec2, WidgetId, Widgets,
     ui::cache::Cache,
     util::{HashMap, HashSet, Vec2, forget_ref},
 };
 
 pub struct UiRenderState {
     pub active: HashMap<WidgetId, ActiveData>,
-    pub layers: PrimitiveLayers,
+    pub layers: DrawLayers,
     pub(super) output_size: Vec2,
     pub cache: Cache,
 
@@ -243,14 +243,11 @@ impl UiRenderState {
     }
 
     pub fn debug_layers(&self) {
-        for ((idx, depth), primitives) in self.layers.iter_depth() {
+        for ((idx, depth), draws) in self.layers.iter_depth() {
             let indent = " ".repeat(depth * 2);
-            let len = primitives.instances().len();
-            print!("{indent}{idx}: {len} primitives");
-            if len >= 1 {
-                print!(" ({})", primitives.instances()[0].binding);
-            }
-            println!();
+            let primitives = draws.primitives.instances().len();
+            let textures = draws.textures.instances().len();
+            println!("{indent}{idx}: {primitives} primitives, {textures} textures");
         }
     }