Add rain and snow to client/server. Tag sectors and linedefs/sidedefs in room editor for texture changes. Fix graphical errors in many rooms.

This implements rain and snow in both the software and hardware renderers. Added an option to the client's Preferences menu (under Special Effects) to turn weather effects off for those who don't like them, or who have a computer that cannot handle them.

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 has 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. To clarify, hardware renderer shows particles coming down only where there is open sky, but I'm pretty sure the software renderer is just drawing them flat on the user's screen (no depth) so this change might not be possible.
Need background sounds for rain/snow.