Implement draw_rain_window and call_draw_rain_func

Implement procedure (0x006842AF) as two methods: draw_rain_window and
call_draw_rain_func.
Avoid using global variable 0x00EE7850 by passing it as a parameter:
draw_rain_func. This global variable was only used by these procedures.
Rain has some visual artifacts in RCT2, specially when using in-game
windows. See this example from the original game: 
![rain-bug](https://cloud.githubusercontent.com/assets/583503/3636735/033ef484-0fdc-11e4-96cb-087a9335cbfa.png)
This implementation matches the original game, and thus has the
same artifacts.