Add MinGW CI job

Author: ChrisThrasher

.github/workflows/ci.yml

@@ -13,6 +13,7 @@ jobs:
         platform:
         - { name: Windows VS2019, os: windows-2019  }
         - { name: Windows VS2022, os: windows-2022  }
+        - { name: Windows MinGW,  os: windows-2022, flags: -GNinja -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++ }
         - { name: Linux GCC,      os: ubuntu-latest }
         - { name: Linux Clang,    os: ubuntu-latest, flags: -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ }
         - { name: MacOS XCode,    os: macos-latest  }