pyscrollgroup + unittest

[JaskRendix]

PR splits out the pyscrollgroup part from #62 and adds unittests. Key changes:

• added a @dataclass called SpriteMeta to organize sprite drawing metadata more cleanly
• replaced raw tuples in draw() with SpriteMeta objects for readability and extensibility
• converted the SpriteMeta list back to tuples before passing to the renderer, keeping compatibility intact
• used getattr() for blendmode to safely handle sprites without it, avoiding exceptions