Add rain and snow to client/server.

This implements rain and snow in both the software and hardware renderers.

Changes to existing code:
The max number of particles that can be displayed has been increased from 128 to 512.
gParticleSystem for effects.sand renamed to gParticleSystemSand.
Room flags for rain and snow added (ROOM_RAINING = 0x10000, ROOM_SNOWING = 0x20000)
RecalcLightAndWeather in room.kod now calls WeatherChanged in room.kod, which sends the same message to all in the plActive list so they can query the room for its weather effects.
Qormas now as pbSnow set to true, and activating it will cause snow in all rooms (but only where there is open sky).

Issues:
Not sure how well this is going to run on slower computers.
Software renderer snow is too fast; I'm not sure how to slow it down.
Still need to modify software renderer rain/snow so that it displays better in rooms with partial ceilings (i.e. outside CV). Hardware mode works fine.