trimesh2d: add library for simple polygon triangulation in linear time

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

libs/trimesh2d/.github/FUNDING.yml

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+github: slimsag

libs/trimesh2d/.github/pull_request_template.md

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+Please send your change to [the main repository](https://github.com/hexops/mach/tree/main/libs/trimesh2d) instead, sorry for the trouble!
+
+This helps us avoid some complex merge conflicts we run into when changes are made to both repositories and history needs to be reconciled. Keeping PRs in just that repository enables us to use `git subtree` to trivially keep the two repositories in sync.
+
+Once your PR is merged over there, it'll automatically sync to this repository.

libs/trimesh2d/.github/workflows/ci.yml

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+name: CI
+on:
+  - push
+  - pull_request
+jobs:
+  x86_64-linux:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Setup Zig
+        run: |
+          sudo apt install xz-utils
+          sudo sh -c 'wget -c https://ziglang.org/builds/zig-linux-x86_64-0.10.0-dev.3952+9e070b653.tar.xz -O - | tar -xJ --strip-components=1 -C /usr/local/bin'
+      - name: x86_64-linux -> aarch64-macos
+        run: zig build test -Dtarget=aarch64-macos.12-none
+      - name: test
+        run: |
+          zig build test
+  x86_64-windows:
+    runs-on: windows-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Install Git
+        run: choco install git
+      - name: Setup Zig
+        run: |
+          $ProgressPreference = 'SilentlyContinue'
+          Invoke-WebRequest -Uri "https://ziglang.org/builds/zig-windows-x86_64-0.10.0-dev.3952+9e070b653.zip" -OutFile "C:\zig.zip"
+          cd C:\
+          7z x zig.zip
+          Add-Content $env:GITHUB_PATH "C:\zig-windows-x86_64-0.10.0-dev.3952+9e070b653\"
+      - name: test
+        run: zig build test
+  x86_64-macos:
+    runs-on: macos-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Setup Zig
+        run: |
+          brew install xz
+          sudo sh -c 'wget -c https://ziglang.org/builds/zig-macos-x86_64-0.10.0-dev.3952+9e070b653.tar.xz -O - | tar -xJ --strip-components=1 -C /usr/local/bin'
+      - name: test
+        run: zig build test