GPU on wgpu's OpenGL backend: renderable depth target, portable shaders, serialised tests

FreeBSD's CI VM (Mesa llvmpipe over GL) failed 22 effectcraft-gpu tests:

- Advanced 3D's camera-depth target was R32Float, which GLES can't render to without
  EXT_color_buffer_float: create_render_pipeline failed validation (release builds only log
  that and draw nothing). The target is now R32Uint holding the f32 bits (renderable on every
  backend, lossless), and GpuContext checks the adapter's format features (render attachment,
  blendable colour) and independent blending up front; when they're missing the render
  pipelines are never built and Advanced 3D renders on the CPU.
- Signed `%` in the shaders: naga emits GLSL `%`, undefined for negative operands, which broke
  the VR effects' horizontal wrap and Cell Pattern's tiling. A shared `imod` never takes `%` of
  a negative number.
- asin/acos: Mesa's are coarse approximations (~1e-4 rad), visible in every lens/sphere/cylinder
  mapping. `asin_p`/`acos_p` (Cephes' asinf polynomial) are f32-accurate on every backend.
- "Could not lock adapter context": the GL backend has one context lock per adapter with a
  timeout, so tests driving devices concurrently panic. Every gpu test helper that hands out a
  device holds one process-wide lock for the calling test; ui_web_job_gpu's two device tests
  share a lock too.
- On GL, up to 1 % of pixels may flip across a decision boundary (division through a
  reciprocal, unfused fma); the GPU-vs-CPU comparisons allow that there only.

Also adds the FreeBSD workflow (from the FreeBSD CI PR) so this branch is checked there.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

.github/workflows/freebsd.yml

@@ -0,0 +1,48 @@
+# Builds and tests the workspace on FreeBSD, inside a FreeBSD VM on a Linux runner
+# (GitHub has no FreeBSD runners). GPU tests skip themselves when the VM has no adapter.
+name: FreeBSD
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    paths:
+      - Cargo.toml
+      - Cargo.lock
+      - "**/Cargo.toml"
+      - .github/workflows/freebsd.yml
+  workflow_dispatch:
+
+concurrency:
+  group: freebsd-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  freebsd:
+    name: FreeBSD 14 (build + test)
+    runs-on: ubuntu-latest
+    timeout-minutes: 300
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build and test in a FreeBSD VM
+        uses: vmactions/freebsd-vm@c46abacb49f09938ca4e1702d15d836285d694cc # v1
+        with:
+          release: "14.3"
+          usesh: true
+          cpu: 4
+          mem: 12288
+          # Nothing to bring back; copying the target dir out of the VM fails on large builds.
+          copyback: false
+          prepare: |
+            pkg install -y git curl pkgconf cmake python3 libxkbcommon wayland libX11 libXcursor libXrandr libXi libxcb mesa-libs vulkan-loader gtk3 fontconfig freetype2 alsa-lib
+            curl -fsSL https://sh.rustup.rs | sh -s -- -y --profile minimal
+          run: |
+            . "$HOME/.cargo/env"
+            # The checkout is synced in under another owner; let tests that call git read it.
+            git config --global --add safe.directory "*"
+            rustc --version
+            CARGO_PROFILE_DEV_DEBUG=0 cargo build --workspace --all-targets --locked
+            CARGO_PROFILE_DEV_DEBUG=0 cargo test --workspace --locked