[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.

qw/source/cl_main.c

@@ -1145,6 +1145,7 @@ CL_SetState (cactive_t state)
 		if (old_state == ca_active) {
 			// leaving active state
 			IN_ClearStates ();
+			CL_ClearState ();
 
 			// Auto demo recorder stops here
 			if (cl_autorecord->int_val && cls.demorecording)