fix rain delay when exiting water

When the particle system updates its internal state noting that it's
been culled, it stops emitting any further particles. Prevent it from
having that knowledge.

v2: Fix off-by-one-frame error following review by @AnyOldName3

CHANGELOG.md

@@ -1,6 +1,7 @@
 0.46.0
 ------
 
+    Bug #4540: Rain delay when exiting water
     Feature #2229: Improve pathfinding AI
     Feature #3442: Default values for fallbacks from ini file
     Task #4686: Upgrade media decoder to a more current FFmpeg API

apps/openmw/mwrender/sky.cpp

@@ -401,11 +401,15 @@ public:
     {
     }
 
-    virtual void operator()(osg::Node* node, osg::NodeVisitor* nv)
+    bool isUnderwater()
     {
         osg::Vec3f eyePoint = mCameraRelativeTransform->getLastEyePoint();
+        return mEnabled && eyePoint.z() < mWaterLevel;
+    }
 
-        if (mEnabled && eyePoint.z() < mWaterLevel)
+    virtual void operator()(osg::Node* node, osg::NodeVisitor* nv)
+    {
+        if (isUnderwater())
             return;
 
         traverse(node, nv);
@@ -1575,6 +1579,8 @@ void SkyManager::update(float duration)
             mRainIntensityUniform->set((float) mWeatherAlpha);
     }
 
+    switchUnderwaterRain();
+
     if (mIsStorm)
     {
         osg::Quat quat;
@@ -1626,6 +1632,15 @@ void SkyManager::updateRainParameters()
     }
 }
 
+void SkyManager::switchUnderwaterRain()
+{
+    if (!mRainParticleSystem)
+        return;
+
+    bool freeze = mUnderwaterSwitch->isUnderwater();
+    mRainParticleSystem->setFrozen(freeze);
+}
+
 void SkyManager::setWeather(const WeatherResult& weather)
 {
     if (!mCreated) return;

apps/openmw/mwrender/sky.hpp

@@ -180,6 +180,7 @@ namespace MWRender
 
         void createRain();
         void destroyRain();
+        void switchUnderwaterRain();
         void updateRainParameters();
 
         Resource::SceneManager* mSceneManager;