Allow jumping when you're stuck on a slope (fixes #4221)

apps/openmw/mwphysics/physicssystem.cpp

@@ -308,6 +308,7 @@ namespace MWPhysics
             float swimlevel = waterlevel + halfExtents.z() - (physicActor->getRenderingHalfExtents().z() * 2 * fSwimHeightScale);
 
             ActorTracer tracer;
+
             osg::Vec3f inertia = physicActor->getInertialForce();
             osg::Vec3f velocity;
 
@@ -320,10 +321,11 @@ namespace MWPhysics
             {
                 velocity = (osg::Quat(refpos.rot[2], osg::Vec3f(0, 0, -1))) * movement;
 
-                if (velocity.z() > 0.f && physicActor->getOnGround() && !physicActor->getOnSlope())
+                if ((velocity.z() > 0.f && physicActor->getOnGround() && !physicActor->getOnSlope())
+                 || (velocity.z() > 0.f && velocity.z() + inertia.z() <= -velocity.z() && physicActor->getOnSlope()))
                     inertia = velocity;
-                else if(!physicActor->getOnGround() || physicActor->getOnSlope())
-                    velocity = velocity + physicActor->getInertialForce();
+                else if (!physicActor->getOnGround() || physicActor->getOnSlope())
+                    velocity = velocity + inertia;
             }
 
             // dead actors underwater will float to the surface, if the CharacterController tells us to do so