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 +1,34 @@
+on:
+  pull_request:
+    paths:
+      - '/packaging/3rdParty-devel/**'
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: docker
+    container:
+      image: forge.a-lec.org/speed-dreams/debian-i686-w64-mingw32:stable
+    steps:
+      - name: Clone repository
+        run: |
+          git clone -q \
+            -b ${{ env.GITHUB_HEAD_REF }} \
+            ${{ env.GITHUB_SERVER_URL }}/${{ env.GITHUB_REPOSITORY }}.git \
+            /src
+      - name: Configure third-party libraries
+        run: cmake -B build -DCMAKE_TOOLCHAIN_FILE=i686-w64-mingw32.cmake
+        working-directory: /src/packaging/3rdParty-devel
+      - name: Build third-party libraries
+        run: cmake --build build -j$(nproc --all)
+        working-directory: /src/packaging/3rdParty-devel
+      - name: Archive third-party libraries
+        run: tar -czf 3rdparty-${{ env.GITHUB_SHA }}.tar.gz 3rdParty/
+        working-directory: /src/packaging/3rdParty-devel/build
+      - name: Upload third-party libraries
+        run: upload-artifact 3rdparty-${{ env.GITHUB_SHA }}.tar.gz
+        working-directory: /src/packaging/3rdParty-devel/build
+        env:
+          URL: ${{ vars.STORE_URL }}
+          USERNAME: ${{ secrets.STORE_USERNAME }}
+          PASSWORD: ${{ secrets.STORE_PASSWORD }}

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

@@ -0,0 +1,37 @@
+on: [pull_request]
+jobs:
+  build:
+    runs-on: docker
+    container:
+      image: forge.a-lec.org/speed-dreams/debian-i686-w64-mingw32:stable
+    steps:
+      - name: Download third-party libraries
+        run: curl -so 3rdparty.tar.gz ${{ vars.THIRDPARTY_URL }}
+      - name: Untar third-party libraries
+        run: tar -xf 3rdparty.tar.gz
+      - name: Clone repository
+        run: |
+          git clone -q \
+            -b ${{ env.GITHUB_HEAD_REF }} \
+            ${{ env.GITHUB_SERVER_URL }}/${{ env.GITHUB_REPOSITORY }}.git \
+            src
+      - name: Configure project
+        run: |
+          cmake -B build \
+            -DSD_ASSUME_DATADIR=1 \
+            -DCMAKE_FIND_ROOT_PATH=${{ env.GITHUB_WORKSPACE}}/3rdParty \
+            -DCMAKE_TOOLCHAIN_FILE=packaging/3rdParty-devel/i686-w64-mingw32.cmake
+        working-directory: src
+      - name: Build project
+        run: cmake --build build -j$(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 }}