CMake: Fix FindSDL2_image.cmake with vcpkg

In case of vcpkg, pkg-config may not be available but a CMake config is.
Prefer CMake config to pkg-config when looking for SDL2_image.

CMake/finders/FindSDL2_image.cmake

@@ -1,26 +1,36 @@
-find_package(PkgConfig)
-pkg_check_modules(PC_SDL2_image QUIET SDL2_image>=2.0.0)
+find_package(SDL2_image CONFIG)
 
-find_path(SDL2_image_INCLUDE_DIR SDL_image.h
-          HINTS ${PC_SDL2_image_INCLUDEDIR} ${PC_SDL2_image_INCLUDE_DIRS})
+if(TARGET SDL2_image::SDL2_image)
+  if(CMAKE_VERSION VERSION_LESS "3.18")
+    # Aliasing local targets is not supported on CMake < 3.18, so make it global.
+    set_target_properties(SDL2_image::SDL2_image PROPERTIES IMPORTED_GLOBAL TRUE)
+  endif()
+  add_library(SDL2::SDL2_image ALIAS SDL2_image::SDL2_image)
+else()
+  find_package(PkgConfig)
+  pkg_check_modules(PC_SDL2_image QUIET SDL2_image>=2.0.0)
+
+  find_path(SDL2_image_INCLUDE_DIR SDL_image.h
+            HINTS ${PC_SDL2_image_INCLUDEDIR} ${PC_SDL2_image_INCLUDE_DIRS})
           
-find_library(SDL2_image_LIBRARY
-             NAMES SDL2_image libSDL2_image
-             HINTS ${PC_SDL2_image_LIBDIR} ${PC_SDL2_image_LIBRARY_DIRS})
+  find_library(SDL2_image_LIBRARY
+               NAMES SDL2_image libSDL2_image
+               HINTS ${PC_SDL2_image_LIBDIR} ${PC_SDL2_image_LIBRARY_DIRS})
 
-include(FindPackageHandleStandardArgs)
-find_package_handle_standard_args(SDL2_image DEFAULT_MSG
-                                  SDL2_image_INCLUDE_DIR SDL2_image_LIBRARY)
+  include(FindPackageHandleStandardArgs)
+  find_package_handle_standard_args(SDL2_image DEFAULT_MSG
+                                    SDL2_image_INCLUDE_DIR SDL2_image_LIBRARY)
 
-if(SDL2_image_FOUND AND NOT TARGET SDL2::SDL2_image)
-  add_library(SDL2::SDL2_image UNKNOWN IMPORTED)
-  set_target_properties(SDL2::SDL2_image PROPERTIES
-                        IMPORTED_LOCATION ${SDL2_image_LIBRARY}
-                        INTERFACE_INCLUDE_DIRECTORIES ${SDL2_image_INCLUDE_DIR})
-endif()
+  if(SDL2_image_FOUND AND NOT TARGET SDL2::SDL2_image)
+    add_library(SDL2::SDL2_image UNKNOWN IMPORTED)
+    set_target_properties(SDL2::SDL2_image PROPERTIES
+                          IMPORTED_LOCATION ${SDL2_image_LIBRARY}
+                          INTERFACE_INCLUDE_DIRECTORIES ${SDL2_image_INCLUDE_DIR})
+  endif()
 
-if(SDL2_image_FOUND)
-  mark_as_advanced(SDL2_image_INCLUDE_DIR SDL2_image_LIBRARY)
-  set(SDL2_image_LIBRARIES ${SDL2_image_LIBRARY})
-  set(SDL2_image_INCLUDE_DIRS ${SDL2_image_INCLUDE_DIR})
+  if(SDL2_image_FOUND)
+    mark_as_advanced(SDL2_image_INCLUDE_DIR SDL2_image_LIBRARY)
+    set(SDL2_image_LIBRARIES ${SDL2_image_LIBRARY})
+    set(SDL2_image_INCLUDE_DIRS ${SDL2_image_INCLUDE_DIR})
+  endif()
 endif()