Add differential testing harness

This runs wgsl shaders and rust shaders and compares the output.
If the output differs, the test fails. Differential testing is better
than snapshot testing or golden file testing as there are no reference
files to get outdated.

Author: LegNeato

.cargo/config.toml

@@ -1,5 +1,6 @@
 [alias]
 compiletest = "run --release -p compiletests --"
+difftest = "run --release -p difftests --"
 
 
 [target.x86_64-pc-windows-msvc]

.github/workflows/ci.yaml

@@ -42,6 +42,13 @@ jobs:
       - if: ${{ runner.os == 'Linux' }}
         name: Linux - Install native dependencies
         run: sudo apt install libwayland-cursor0 libxkbcommon-dev libwayland-dev
+      - if: ${{ runner.os == 'Linux' }}
+        name: Install xvfb, llvmpipe and lavapipe
+        run: |
+          sudo apt-get update -y -qq
+          sudo add-apt-repository ppa:kisak/turtle -y
+          sudo apt-get update
+          sudo apt install -y xvfb libgl1-mesa-dri libxcb-xfixes0-dev mesa-vulkan-drivers
       # cargo version is a random command that forces the installation of rust-toolchain
       - name: install rust-toolchain
         run: cargo version
@@ -64,6 +71,10 @@ jobs:
         if: ${{ matrix.target != 'aarch64-linux-android' }}
         run: cargo run -p compiletests --release --no-default-features --features "use-installed-tools" -- --target-env vulkan1.1,vulkan1.2,spv1.3
 
+      - name: difftest
+        if: ${{ matrix.target != 'aarch64-linux-android' }}
+        run: cargo run -p difftests --release --no-default-features --features "use-installed-tools"
+
       - name: workspace test
         if: ${{ matrix.target != 'aarch64-linux-android' }}
         run: cargo test --release
@@ -145,8 +156,10 @@ jobs:
         run: cargo fetch --locked
       - name: Rustfmt
         run: cargo fmt --all -- --check
-      - name: Rustfmt tests
-        run: rustfmt --check tests/ui/**/*.rs
+      - name: Rustfmt compiletests
+        run: shopt -s globstar && rustfmt --check tests/compiletests/ui/**/*.rs
+      - name: Rustfmt difftests
+        run: cargo fmt --check --all --manifest-path tests/difftests/tests/Cargo.toml
       - name: Check docs are valid
         run: RUSTDOCFLAGS=-Dwarnings cargo doc --no-deps
       - name: Check docs for `spirv-std` and `spirv-builder` on stable (for docs.rs)