🔨 AppVeyor: Switch to Visual Studio 2019

SDL_audiolib uses some C++17 features not supported in VS2019

appveyor.yml

@@ -2,7 +2,7 @@ version: 1.0.{build}
 
 pull_requests:
   do_not_increment_build_number: true
-image: Visual Studio 2017
+image: Visual Studio 2019
 cache: c:\tools\vcpkg\installed\
 
 install:
@@ -13,7 +13,7 @@ install:
   - vcpkg install --recurse fmt:x64-windows sdl2:x64-windows sdl2-mixer:x64-windows sdl2-ttf:x64-windows libsodium:x64-windows
 
 before_build:
-  - cmake -G "Visual Studio 15 2017" -A x64 -DNIGHTLY_BUILD=ON -DCMAKE_TOOLCHAIN_FILE=c:/tools/vcpkg/scripts/buildsystems/vcpkg.cmake .
+  - cmake -G "Visual Studio 16 2019" -A x64 -DNIGHTLY_BUILD=ON -DCMAKE_TOOLCHAIN_FILE=c:/tools/vcpkg/scripts/buildsystems/vcpkg.cmake .
 
 build:
   project: $(APPVEYOR_BUILD_FOLDER)\$(APPVEYOR_PROJECT_NAME).sln