Add GitHub Actions Windows build steps.

This will allows us to determine whetever any megasource changes causes
build failure in LOVE later on, rather than rerunning LOVE jobs
everytime.

.github/workflows/deps-test.yml

@@ -0,0 +1,28 @@
+name: build
+on: [push, pull_request]
+
+jobs:
+  windows:
+    runs-on: windows-latest
+    strategy:
+      matrix:
+        platform: [Win32, x64]
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+      with:
+        path: megasource
+    - name: Checkout LÖVE
+      uses: actions/checkout@v2
+      with:
+        path: megasource/libs/love
+        repository: love2d/love
+        ref: master
+    - name: Configure
+      shell: cmd
+      env:
+        PLATFORM: ${{ matrix.platform }}
+      run: cmake -Bbuild -Hmegasource -T v142 -A %PLATFORM% -DCMAKE_INSTALL_PREFIX=%CD%\install
+    - name: Install
+      shell: cmd
+      run: cmake --build build --config Release --target install -j2