Fix missing defaults for `fx_rain` and broken weather zone creation

[TwelveEyes]

lightrain and wind added to fx_rain since it was missing defaults if it was created without any spawn flags (since the JO weather system doesn't use spawn flags). JO's rain system always has random wind, unlike JA's rain system. Enabling wind is close enough for this purpose.

JO code: https://github.com/grayj/Jedi-Outcast/blob/master/code/game/g_fx.cpp#L276-L287 https://github.com/grayj/Jedi-Outcast/blob/master/code/renderer/tr_worldeffects.cpp#L1986-L2003

Auto generated weather zones were also not being created properly beyond first map load because of bad operation placement, resulting in weather effects clipping through geometry. Currently, if a cache file is found and mWeatherZones is empty (not all maps have pre-placed weather zones, for example JO's yavin4) then mWeatherZones will never be filled, and the point data from the cached file is useless. Weather zones should always be generated if mWeatherZones is empty (whether or not a cache file is found is irrelevant), as the cache file only contains point data, not weather zones themselves.

Fixes #1104

[ensiform]

Does modifying the jk2 game code make most sense? Or would it make more sense to make changes to weather code for jk2 variant only?

[TwelveEyes]

Does modifying the jk2 game code make most sense? Or would it make more sense to make changes to weather code for jk2 variant only?

I think adding 1-2 lines to the game fx entities makes more sense than adding 10+ lines to the weather system to get the same outcome.