Players stay in environment even when dead, damage flash and fall damage fixes

Don't set m_removed on dead players (dead players are indicated by hp == 0). Local
damage flash is shown whatever the cause was (even from Lua set_hp). PlayerCAO
damage flash matches duration of local damage flash. Fall damage is dealt much more consistently (this is done by disallowing jumping when speed.Y is very negative, up to now jumping could sometimes negate fall damage)

src/serverremoteplayer.h

@@ -90,7 +90,6 @@ public:
 	int m_wield_index;
 	bool m_inventory_not_sent;
 	bool m_hp_not_sent;
-	bool m_respawn_active;
 	bool m_is_in_environment;
 	// Incremented by step(), read and reset by Server
 	float m_time_from_last_punch;