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 game
windows. This implementation matches the original game, and thus has the
same artifacts.
Merge pull request #217 from marcotc/rain

Implement draw_rain_window and call_draw_rain_func