Draw each primitive with its own pipeline, registered rather than declared

The `primitives!` macro, `PrimitiveData`, `PrimitiveVec`,
`PrimitiveBuffers`, the `Primitive` trait and the shader's dispatch
switch are gone. A primitive is now a registration: its WGSL and,
implicitly, the size of the entry that WGSL reads. Everything else --
its instance list, its free list, its buffers, its bind group and its
pipeline -- follows from that, so adding one is a `register` call and a
shader file, with nothing per-type to remember and no cross-type
dispatch to extend.

Nothing dispatches dynamically. Push, free, renumber, upload and draw
are identical for every primitive; what differs is the entry size and
the pipeline, which are data. So `InstanceList` carries a runtime
stride and its instances' data as bytes, and one concrete type serves
every primitive and the textures. Measured against a typed list it
costs 0.2ns per write, where a trait object costs 1.6ns.

Because each type has its own list, an instance's index is also its
data index: `@builtin(instance_index)` replaces the `idx` field, the
`binding` field goes with the switch, and `PrimitiveInstance` drops from
28 bytes to 20. `PrimitiveVec`'s free list merges into the instance
list's, so an instance and its data are freed by one `swap_remove`
rather than two arenas kept in step.

`shader.wgsl` becomes `shader/prelude.wgsl` plus one file per primitive.
The prelude carries the window, masks, sampled texture, vertex shader
and `masked()`, and is compiled ahead of each primitive's own source --
which is also what a caller's own primitive would be. Masks move back
into group 0 beside the window uniform, since every pipeline shares one
layout.

Within a layer, types now draw in registration order: a rect under a
glyph under a texture. Order within a layer was never meaningful --
freeing an instance swaps another into its place -- so this replaces an
accident with a defined order, and backgrounds land under their content.

Verified with a headless run per case: text over its own rect and an
image over its own rect in one layer, a masked stack clipping, the
text-layout tab, and adding three images and deleting one.

src/widget/rect.rs

@@ -29,12 +29,15 @@ impl Rect {
 
 impl Widget for Rect {
     fn draw(&mut self, painter: &mut Painter) {
-        painter.primitive(RectPrimitive {
-            color: self.color,
-            radius: self.radius,
-            thickness: self.thickness,
-            inner_radius: self.inner_radius,
-        });
+        painter.primitive(
+            RECT,
+            RectPrimitive {
+                color: self.color,
+                radius: self.radius,
+                thickness: self.thickness,
+                inner_radius: self.inner_radius,
+            },
+        );
     }
 
     fn desired_width(&mut self, _: &mut SizeCtx) -> Len {

src/widget/text/edit.rs

@@ -73,6 +73,7 @@ impl Widget for TextEdit {
             let size = vec2(rect.width() as f32, rect.height() as f32);
             let top_left = vec2(rect.x0 as f32, rect.y0 as f32);
             painter.primitive_within(
+                RECT,
                 RectPrimitive::color(Color::SKY),
                 size.align(Align::TOP_LEFT).offset(top_left).within(&region),
             );
@@ -82,6 +83,7 @@ impl Widget for TextEdit {
         let size = vec2(caret.width() as f32, caret.height() as f32);
         let top_left = vec2(caret.x0 as f32, caret.y0 as f32);
         painter.primitive_within(
+            RECT,
             RectPrimitive::color(Color::WHITE),
             size.align(Align::TOP_LEFT).offset(top_left).within(&region),
         );