remove webp from source tree and fix some bugs

external/libwebp/CMakeLists.txt

@@ -1,6 +1,16 @@
 cmake_minimum_required(VERSION 3.9 FATAL_ERROR)
 
-set(WEBP_LINK_STATIC ON CACHE INTERNAL "")
+include(FetchContent)
 
-add_subdirectory(./libwebp EXCLUDE_FROM_ALL)
+if (NOT TARGET libwebp)
+	set(WEBP_LINK_STATIC ON CACHE INTERNAL "")
+	FetchContent_Declare(libwebp
+		GIT_REPOSITORY https://github.com/webmproject/libwebp
+		GIT_TAG v1.3.2
+		FIND_PACKAGE_ARGS
+	)
+	FetchContent_MakeAvailable(libwebp)
+endif()
+
+#add_subdirectory(./libwebp EXCLUDE_FROM_ALL)