Draw the glyph atlas as an array texture and images with their own bind groups

The renderer bound every texture through one
`binding_array<texture_2d<f32>>` indexed per primitive. That needs
`VK_EXT_descriptor_indexing`, which a real share of Android GPUs do not
have, so the shape did not run there at all.

Split the two things being bound, since they want opposite treatment:

- Glyph atlas pages become layers of one `texture_2d_array`. A glyph
  primitive carries a layer rather than a view/sampler index pair, and a
  layer index is an ordinary sampling operand -- no extension. Growing the
  atlas recreates the array with headroom and copies the old layers across
  GPU-side.
- A standalone image gets its own texture and its own bind group, and
  draws in its own call. It no longer needs a per-instance entry in
  `PrimitiveData` at all: the bind group has already picked the texture.

`Primitives` therefore keeps images in a list of their own, with
`PrimitiveChange::is_image` saying which list a renumbering belongs to --
the two have independent index spaces, so `(layer, inst_idx)` alone would
collide between them.

Verified on this machine's real GPU (Venus onto an RX 7900 XT, confirmed
by the loaded ICD rather than assumed): the `tabs` example renders
byte-identical screenshots before and after, both for a text-and-rect tab
and for one holding a standalone image.

core/src/render/atlas.rs

@@ -5,7 +5,12 @@ use crate::{
 use image::RgbaImage;
 use swash::scale::image::{Content, Image};
 
-const PAGE: u32 = 1024;
+/// Side of one atlas page, in pixels. 1024 is 4 MB at RGBA8 -- enough for a
+/// few thousand glyphs at UI sizes, and small enough that a page nobody fills
+/// is not a big waste. Also the fixed width/height of every layer of the
+/// shared array texture in `render::texture` -- `pub(crate)` so that module
+/// can size it without a second constant to keep in sync.
+pub(crate) const PAGE: u32 = 1024;
 
 /// Transparent margin kept around every glyph, so that sampling one cannot
 /// pick up its neighbour along a shared edge.
@@ -35,8 +40,8 @@ pub struct GlyphEntry {
     pub width: u32,
     pub height: u32,
     pub is_colored: bool,
-    pub view_idx: u32,
-    pub sampler_idx: u32,
+    /// The atlas array layer this glyph's page occupies.
+    pub layer: u32,
 }
 
 impl GlyphEntry {
@@ -120,8 +125,7 @@ impl GlyphAtlas {
             width: w,
             height: h,
             is_colored: matches!(image.content, Content::Color),
-            view_idx: page.handle.primitive().view_idx,
-            sampler_idx: page.handle.primitive().sampler_idx,
+            layer: page.handle.layer(),
         };
         self.entries.insert(key, Some(entry));
         Some(entry)
@@ -137,7 +141,7 @@ impl GlyphAtlas {
             return (i, x, y);
         }
 
-        let handle = textures.add(RgbaImage::new(PAGE, PAGE));
+        let handle = textures.add_page(RgbaImage::new(PAGE, PAGE));
         self.pages.push(Page {
             handle,
             x: PAD + w + PAD,