Update jsoncpp from version 0.5.0 to 1.9.5

Aside from including a ton of bug fixes, this should fix an issue in the
CI pipeline when building with system libraries that was caused by a
minor incompatibility between 0.5.0 and less ancient versions.

Also, switch to the amalgamated source, because it makes it easier to
keep it up to date.
https://github.com/open-source-parsers/jsoncpp/wiki/Amalgamated-(Possibly-outdated)

src/RageUtil.cpp

@@ -2582,7 +2582,7 @@ int LuaFunc_JsonEncode(lua_State* L)
 				Json::Value array(Json::arrayValue);
 				array.resize(len);
 
-				for (size_t i = 0; i < len; i++)
+				for (int i = 0; i < len; i++)
 				{
 					lua_rawgeti(L, index, i + 1);
 					array[i] = convert(-1);
@@ -2662,7 +2662,7 @@ int LuaFunc_JsonDecode(lua_State* L)
 	bool ok = reader.parse(std::string(data, datalen), root, true);
 	if (!ok)
 	{
-		std::string error = reader.getFormatedErrorMessages();
+		std::string error = reader.getFormattedErrorMessages();
 		luaL_error(L, "failed to parse JSON: %s", error.c_str());
 	}