HUD hit indicator: implement a 2d directional hit indicator around the screen center...

HUD hit indicator: implement a 2d directional hit indicator around the screen center when a player hits you

* for now only 1 indicator at a time
* starts to fade out as soon as it appears
* the higher the damage, the bigger the indicator size
* damage is accumulated
* hidden when the attacker is not visible
* disabled if chase_active is enabled
* shown while spectating a player too
* implemented as an extension of damagetext (requires sv_damagetext >= 2)

Extra: added cl_damagetext_received (disabled by default) that allows displaying damage text when you receive damage too