Let each primitive record its own draws

`draw` had three branches, one per shader, which is the dynamic dispatch this
was asking for. A primitive now brings a `PrimitiveRender`: it states the
layout its shader reads, uploads whatever it owns, and records its own draws.
`GlyphRender` owns the atlas and binds it once for a list; `ImageRender` owns
the images and binds one per instance; the default owns nothing and draws them
all in one call. The renderer sets the pipeline, the shared group, the list's
data and the vertex buffer, and knows nothing else about what it is drawing.

Measured before committing to it, since dispatch per list is the cost. Wall
time on this machine swings 2x between runs of one binary, so the comparison
is instructions retired, which is stable to 0.1%: at 256 layers drawing 8
rects, 8 glyphs and 2 images each, 7.4074e9 against 7.4145e9, and at 1024
layers 27.467e9 against 27.498e9. Both are 0.1%, which is 6 instructions per
list drawn -- one indirect call. Recording a list into the pass costs wgpu
about 5,400.

`tests/draw_cost.rs` is that measurement, kept.

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

examples/tabs/main.rs

@@ -215,7 +215,7 @@ impl DefaultAppState for Client {
             "widgets: {}\nactive: {}\ntextures: {}",
             rsc.widgets().len(),
             render.active_widgets(),
-            self.ui_state.renderer.ui.texture_count(),
+            rsc.ui().textures.count(),
         );
         if new != *rsc.widgets()[self.info].content {
             *rsc.widgets_mut()[self.info].content = new;