Use cast instead of untyped for _getTarget().

Fixed compilation issue for Java target:


```haxe
class Test {
	static function main() {
		tweenx909.TweenX.to({}, {});
	}
}
```

The above produced the following compilation error:
```
haxelib run hxjava hxjava_build.txt --haxe-version 3101
javac "-sourcepath" "src" "-d" "obj" "-g" "@cmd"
src/tweenx909/advanced/StandardTweenX.java:115: error: incompatible types
		return this;
		       ^
  required: T
  found:    StandardTweenX<T>
  where T is a type-variable:
    T extends Object declared in class StandardTweenX
1 error
Compilation error
Native compilation failed
Error: Build failed
[Finished in 2.1s with exit code 1]
[cmd: ['haxe', 'build.hxml']]
[dir: /Users/andy/Documents/workspace/TestHaxe]
[path: /usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin]
```