Separate atlas pages from textures, and draw both through one instance list

Rework of the review on #11. Pages and standalone images were one
`Textures` manager separated by a `TextureKind` tag, and images were a
second instance list beside `Primitives::instances`. The tag forced
`image_index()`/`layer()` to panic on the wrong kind of handle, and the
second list forced an `is_image` branch through `free`, `region_mut`,
`apply_free` and `PrimitiveChange`.

Pages are now their own thing. `GlyphAtlas` owns its page images
outright and hands the renderer dirty rectangles; `GpuPages` owns the
array texture they upload to. `Textures` is standalone images only, so
`TextureHandle` has one kind, `slot()` cannot be wrong, and nothing
needs a free list that skips pages. `GlyphAtlas::insert` no longer takes
a `Textures`, which drops that parameter from `TextData::render` and
`SizeCtx` too.

Images go back through the one instance list. A texture instance is an
ordinary `PrimitiveInstance` whose `idx` names a texture rather than a
group-1 entry, which `PrimitiveHandle::data_idx: Option` records.
`RenderLayer::plan_draws` batches the layer's instances into runs
sharing a bind group, so a ui with no images still plans a single draw,
and an image draws in instance order rather than on top of its layer.

Group 2 is now one `texture_2d_array` and a sampler, bound per run: the
atlas for rects and glyphs, or one image viewed as an array of one. That
removes the second texture binding and the 1x1 null view that had to
fill it. Masks move to group 3, so resizing that buffer no longer stales
every texture bind group, and `GpuTextures` no longer reports whether
the caller must rebuild one.

`GlyphPrimitive` drops its manual pad: the WGSL struct now declares the
uvs as scalars, which matches the Rust layout exactly. `#[repr(C,
align(8))]` would have left real padding bytes, which `bytemuck::Pod`
forbids.

Verified with a headless run of the `tabs` example and of a scratch
example mixing images, rects, glyphs and a mask in one layer; 52 glyphs
at size 300 grew the atlas array from 1 to 4 layers with every earlier
page still sampling correctly.

core/src/primitive/layer.rs

@@ -133,15 +133,15 @@ impl PrimitiveLayers {
         self[h.layer].free(h)
     }
 
-    pub fn write_image(
+    pub fn write_texture(
         &mut self,
         layer: LayerId,
         id: WidgetId,
-        texture_idx: u32,
+        texture: u32,
         region: UiRegion,
         mask_idx: MaskIdx,
     ) -> PrimitiveHandle {
-        self[layer].write_image(layer, id, texture_idx, region, mask_idx)
+        self[layer].write_texture(layer, id, texture, region, mask_idx)
     }
 }