[renderer] Make R_RenderView private

This is actually a better solution to the renderer directly accessing
client code than provided by 7e078c7f9c.

Essentially, V_RenderView should not have been calling R_RenderView, and
CL_UpdateScreen should have been calling V_RenderView directly. The
issue was that the renderers expected the world entity model to be valid
at all times. Now, R_RenderView checks the world entity model's validity
and immediately bails if it is not, and R_ClearState (which is called
whenever the client disconnects and thus no longer has a world to
render) clears the world entity model. Thus R_RenderView can (and is)
now called unconditionally from within the renderer, simplifying
renderer-specific variants.

nq/include/client.h

@@ -328,7 +328,6 @@ void CL_NewTranslation (int slot, struct skin_s *skin);
 void V_StartPitchDrift (void);
 void V_StopPitchDrift (void);
 
-void V_RenderView (void);
 void V_UpdatePalette (void);
 void V_Register (void);
 void V_ParseDamage (void);