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/primitive/layer.rs

@@ -1,6 +1,7 @@
 use std::ops::{Index, IndexMut};
 
 use crate::{
+    UiRegion, WidgetId,
     render::{MaskIdx, Primitive, PrimitiveHandle, PrimitiveInst, Primitives},
     util::to_mut,
 };
@@ -131,6 +132,17 @@ impl PrimitiveLayers {
     pub fn free(&mut self, h: &PrimitiveHandle) -> MaskIdx {
         self[h.layer].free(h)
     }
+
+    pub fn write_image(
+        &mut self,
+        layer: LayerId,
+        id: WidgetId,
+        texture_idx: u32,
+        region: UiRegion,
+        mask_idx: MaskIdx,
+    ) -> PrimitiveHandle {
+        self[layer].write_image(layer, id, texture_idx, region, mask_idx)
+    }
 }
 
 impl<T: Default> Default for Layers<T> {