Take a primitive's kind from its type, and keep images out of the rest

A `Primitive` now carries its own WGSL, and `PrimitiveRegistry` keys ids by
`TypeId`, so `Painter::primitive` takes only the value and the `RECT`,
`GLYPH` and `TEXTURE` constants are gone. Registering is what a first draw
does; the built-ins are seeded up front so first-draw order cannot decide
anything about them.

What a primitive samples is no longer something every registration states.
The glyph atlas and the one sampler moved into the shared group, which is
where a mask texture would go too, so a rect's pipeline has no texture in
its layout at all. Only a primitive whose type sets `TEXTURE` gets an image
group, and that is also what records the slot at write time -- so nothing
reads a `u32` back out of the instance payload.

Verified on the headless rig: the tabs example, two images added at runtime,
an image alone in a layer, and text spanning a four-layer atlas after the
array grew twice.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/src/ui/painter.rs

@@ -2,8 +2,8 @@ use crate::{
     Axis, Len, RenderedText, Size, SizeCtx, StrongWidget, TextAttrs, TextBuffer, TextData,
     TextureHandle, UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2, Widget, WidgetId,
     render::{
-        GLYPH, GlyphPrimitive, Mask, MaskIdx, Primitive, PrimitiveHandle, PrimitiveInst,
-        PrimitiveKind, TEXTURE, TexturePrimitive,
+        GlyphPrimitive, Mask, MaskIdx, Primitive, PrimitiveHandle, PrimitiveInst, PrimitiveKind,
+        TexturePrimitive,
     },
     util::Vec2,
 };
@@ -23,12 +23,14 @@ pub struct Painter<'a> {
 }
 
 impl<'a> Painter<'a> {
-    fn primitive_at<P: Primitive>(
-        &mut self,
-        kind: PrimitiveKind<P>,
-        primitive: P,
-        region: UiRegion,
-    ) {
+    fn primitive_at<P: Primitive>(&mut self, primitive: P, region: UiRegion) {
+        let kind = self.rsc.ui_mut().primitives.kind::<P>();
+        self.write(kind, primitive, region);
+    }
+
+    /// For a caller with many of one primitive to write, since looking the kind
+    /// up is per type rather than per instance.
+    fn write<P: Primitive>(&mut self, kind: PrimitiveKind<P>, primitive: P, region: UiRegion) {
         let h = self.state.layers.write(
             self.layer,
             PrimitiveInst {
@@ -51,17 +53,12 @@ impl<'a> Painter<'a> {
     }
 
     /// Writes a primitive to be rendered
-    pub fn primitive<P: Primitive>(&mut self, kind: PrimitiveKind<P>, primitive: P) {
-        self.primitive_at(kind, primitive, self.region)
+    pub fn primitive<P: Primitive>(&mut self, primitive: P) {
+        self.primitive_at(primitive, self.region)
     }
 
-    pub fn primitive_within<P: Primitive>(
-        &mut self,
-        kind: PrimitiveKind<P>,
-        primitive: P,
-        region: UiRegion,
-    ) {
-        self.primitive_at(kind, primitive, region.within(&self.region));
+    pub fn primitive_within<P: Primitive>(&mut self, primitive: P, region: UiRegion) {
+        self.primitive_at(primitive, region.within(&self.region));
     }
 
     pub fn set_mask(&mut self, region: UiRegion) {
@@ -104,7 +101,6 @@ impl<'a> Painter<'a> {
     pub fn texture_at(&mut self, handle: &TextureHandle, region: UiRegion) {
         self.textures.push(handle.clone());
         self.primitive_at(
-            TEXTURE,
             TexturePrimitive {
                 slot: handle.slot(),
             },
@@ -123,6 +119,7 @@ impl<'a> Painter<'a> {
     }
 
     pub fn glyphs(&mut self, text: &RenderedText, origin: UiRegion) {
+        let kind = self.rsc.ui_mut().primitives.kind::<GlyphPrimitive>();
         for glyph in text.glyphs.iter() {
             let mut region = origin;
             region.x.end = region.x.start;
@@ -130,8 +127,8 @@ impl<'a> Painter<'a> {
             let mut region = region.offset(UiVec2::abs(glyph.offset));
             region.x.end = region.x.start + UiScalar::abs(glyph.entry.width as f32);
             region.y.end = region.y.start + UiScalar::abs(glyph.entry.height as f32);
-            self.primitive_at(
-                GLYPH,
+            self.write(
+                kind,
                 GlyphPrimitive {
                     uv_min: glyph.entry.uv_min,
                     uv_max: glyph.entry.uv_max,