iris: a one-layer glyph atlas is a GL_TEXTURE_2D, so every glyph drew as a box

The emulator was blamed for two days for what is iris's own defect on any
GL adapter. `GpuTextures::new` created the atlas `texture_2d_array` with
one layer; wgpu-hal picks the GL target from the descriptor alone
(`gles::Texture::get_info_from_desc`, `(false, 1) => TEXTURE_2D`), so the
shader's `sampler2DArray` was handed a `GL_TEXTURE_2D`, the unit was
incomplete, every `textureSample` returned (0,0,0,1), and `draw_glyph`'s
`color.a *= texel.a` painted the whole glyph quad.

`MIN_ARRAY_LAYERS = 2`, with the account at `create_array_texture` and a
`debug_assert!` there. Vulkan -- the phone's build and the desktop's
default backend -- was never affected.

`force-gles` now switches the desktop backend too, so the GLES path is
reproducible on a machine with a real GPU in seconds rather than only
through an APK: that is how this was found, with two shader probes
showing the sample was exactly (0,0,0,1).

docs/IRIS_TODO.md

@@ -609,17 +609,20 @@ agent ticks it here with the evidence.
   format rather than the palette -- but it makes the desktop build
   useless as a colour reference, which is exactly what P1a needed it for
   when the emulator could not draw glyphs.
-- [ ] **The emulator cannot draw iris's glyphs.** Under `-gpu host` with
-  Vulkan disabled (Mesa 26.2.2 / virgl -- what `emu` does on this
-  machine) every character renders as a solid filled box: the atlas
-  sample's alpha reads 1, which is what an incomplete GL texture returns
-  (0,0,0,1). Not new (`20303e0` does it too) and not the platform's
-  (Compose draws text perfectly on the same AVD in the same minute).
-  Enabling host Vulkan still dies at boot in gfxstream, and
-  `EMU_GPU=software` gives SwiftShader Vulkan on which iris **SIGSEGVs
-  in `surface_changed`**. Either of the last two would restore
-  appearance testing on Android; today it has to be done on the desktop
-  backend or on Iris's phone.
+- [x] **Every glyph was a solid box on the GLES backend -- iris's bug,
+  not the emulator's.** Fixed 2026-09-06. The atlas is one
+  `texture_2d_array` and `GpuTextures::new` created it with **one
+  layer**; wgpu-hal picks the GL target from the descriptor
+  (`(false, 1) => TEXTURE_2D`), so under GLES that array was a
+  `GL_TEXTURE_2D` bound to the shader's `sampler2DArray`, the unit was
+  incomplete, every `textureSample` returned (0,0,0,1), and
+  `draw_glyph`'s `color.a *= texel.a` filled the quad. `MIN_ARRAY_LAYERS
+  = 2` in `iris/core/src/render/texture.rs`, with a `debug_assert!` at
+  `create_array_texture`. Vulkan (the phone, the desktop's default
+  backend) was never affected. Reproduce the class in seconds without an
+  emulator: `iris`'s `force-gles` feature now switches the **desktop**
+  backend too -- `./run-headless.sh transcript --shot /tmp/x.png -- -p
+  transcript-ui --features iris/force-gles`.
 
 ## Build (for the port)