Introduce w64-mingw32 workflows

- build-3rdparty.yml cross-builds all third-party dependencies for
Windows. This workflow is meant to be run only sparingly, since
it fetches source code from many repositories and takes a long time to
build.

- build-w64-mingw32.yml cross-builds the game engine and tools for
Windows. It requires a pre-existing build of the third-party libraries.

.forgejo/workflows/build-3rdparty.yml

@@ -0,0 +1,24 @@
+on:
+  pull_request:
+    paths:
+      - '/packaging/3rdParty-devel/**'
+
+jobs:
+  build:
+    runs-on: docker
+    container:
+      image: forge.a-lec.org/speed-dreams/x86_64-w64-mingw32-docker:stable
+    steps:
+      - name: Clone repository
+        run: |
+          git clone \
+            -b ${{ env.GITHUB_HEAD_REF }} \
+            --depth 1 \
+            '${{ env.GITHUB_SERVER_URL }}/${{ env.GITHUB_REPOSITORY }}.git' \
+            /src
+      - name: Configure third-party libraries
+        run: cmake -B build -DCMAKE_TOOLCHAIN_FILE=x86_64-w64-mingw32.cmake
+        working-directory: /src/packaging/3rdParty-devel
+      - name: Build third-party libraries
+        run: cmake --build build -j$(nproc --all)
+        working-directory: /src/packaging/3rdParty-devel

.forgejo/workflows/build-w64-mingw32.yml

@@ -0,0 +1,39 @@
+on: [pull_request]
+jobs:
+  build:
+    runs-on: docker
+    container:
+      image: forge.a-lec.org/speed-dreams/i686-w64-mingw32-docker:stable
+    steps:
+      - name: Download 3rdParty libraries
+        run: |
+          curl -O 3rdparty.zip "${{ vars.3RDPARTY_URL }}"
+      - name: Unzip 3rdParty libraries
+        run: |
+          unzip 3rdparty.zip -d /3rdparty
+      - name: Clone repository
+        run: |
+          git clone \
+            -b ${{ env.GITHUB_HEAD_REF }} \
+            --depth 1 \
+            ${{ env.GITHUB_SERVER_URL }}/${{ env.GITHUB_REPOSITORY }}.git \
+            /src
+      - name: Configure project
+        run: cmake -B build \
+          -DSD_ASSUME_DATADIR=1 \
+          -DCMAKE_FIND_ROOT_PATH=/3rdparty \
+          -DCMAKE_TOOLCHAIN_FILE=packaging/3rdParty-devel/i686-w64-mingw32.cmake
+        working-directory: /src
+      - name: Build project
+        run: cmake --build build -j$(nproc --all)
+        working-directory: /src
+      - name: Build installer
+        run: cpack -G NSIS
+        working-directory: /src/build
+      - name: Upload installer
+        run: upload-artifact *.exe
+        working-directory: /src/build
+        env:
+          URL: ${{ vars.STORE_URL }}
+          USERNAME: ${{ secrets.STORE_USERNAME }}
+          PASSWORD: ${{ secrets.STORE_PASSWORD }}