Created named address RCT2_ADDRESS_SCENARIO_TICKS

* 0x00F663AC is now named RCT2_ADDRESS_SCENARIO_TICKS. This variable
starts at an arbitrary value, defined by the current scenario, and
increments by 1 on every game tick.
* Also, added meaningful name to some variables in draw_light_rain and
draw_heavy_rain.
Update gfx_draw_rain declaration to match definition
Merge pull request #363 from marcotc/rain

Created named address RCT2_ADDRESS_SCENARIO_TICKS