ideas for rain imporvments

[Frostwithasideofsalt]

ideas to improve rain;

• have option to make it not interact with tiles
• have the sprite be redrawn
• have option to set size of rain particles
• fix issues brought up in #2606

[HybridDog]

It could also be possible to implement Photorealistic Rendering of Rain Streaks but this may not fit since SuperTux is not photo-realistic.