AppVeyor: use newer MinGW-w64 toolchain

Otherwise, libSDL2 isn't compatible with an elder one

.appveyor.yml

@@ -71,21 +71,25 @@ environment:
       COMPILER_FAMILY: MinGW
       GENERATOR: "MinGW Makefiles"
       PLATFORM: Win32
+      APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2015"
     - BUILD_TYPE: Release
       COMPILER: MinGW-w32
       COMPILER_FAMILY: MinGW
       GENERATOR: "MinGW Makefiles"
       PLATFORM: Win32
+      APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2015"
     - BUILD_TYPE: Debug
       COMPILER: MinGW-w64
       COMPILER_FAMILY: MinGW
       GENERATOR: "MinGW Makefiles"
       PLATFORM: x64
+      APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2015"
     - BUILD_TYPE: Release
       COMPILER: MinGW-w64
       COMPILER_FAMILY: MinGW
       GENERATOR: "MinGW Makefiles"
       PLATFORM: x64
+      APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2015"
     - BUILD_TYPE: Debug
       COMPILER: MSVC15
       COMPILER_FAMILY: MSVC
@@ -180,8 +184,8 @@ build_script:
   - md build-%COMPILER%-%BUILD_TYPE%-%PLATFORM%
   - cd build-%COMPILER%-%BUILD_TYPE%-%PLATFORM%
   - if [%COMPILER%]==[MinGW] set PATH=C:\MinGW\bin;%PATH:C:\Program Files\Git\usr\bin;=%
-  - if [%COMPILER%]==[MinGW-w32] set PATH=C:\mingw-w64\i686-6.3.0-posix-dwarf-rt_v5-rev1\mingw32\bin;%PATH:C:\Program Files\Git\usr\bin;=%
-  - if [%COMPILER%]==[MinGW-w64] set PATH=C:\mingw-w64\x86_64-6.3.0-posix-seh-rt_v5-rev1\mingw64\bin;%PATH:C:\Program Files\Git\usr\bin;=%
+  - if [%COMPILER%]==[MinGW-w32] set PATH=C:\mingw-w64\i686-8.1.0-posix-dwarf-rt_v6-rev0\mingw32\bin;%PATH:C:\Program Files\Git\usr\bin;=%
+  - if [%COMPILER%]==[MinGW-w64] set PATH=C:\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0\mingw64\bin;%PATH:C:\Program Files\Git\usr\bin;=%
   - if NOT [%COMPILER_FAMILY%]==[MSVC2] cmake -G "%GENERATOR%" -DCMAKE_BUILD_TYPE=%BUILD_TYPE% -DCMAKE_PREFIX_PATH=. -DCMAKE_INSTALL_PREFIX=SDL-Mixer-X -DDOWNLOAD_AUDIO_CODECS_DEPENDENCY=YES %EXTRA_CMAKE_FLAGS% ..
   - if [%COMPILER_FAMILY%]==[MSVC2] cmake -G "%GENERATOR%" -A %PLATFORM% -DCMAKE_BUILD_TYPE=%BUILD_TYPE% -DCMAKE_PREFIX_PATH=. -DCMAKE_INSTALL_PREFIX=SDL-Mixer-X -DDOWNLOAD_AUDIO_CODECS_DEPENDENCY=YES %EXTRA_CMAKE_FLAGS% ..
   - if [%COMPILER_FAMILY%]==[MinGW] cmake --build . --config %BUILD_TYPE% -- -j 2