[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.

include/r_internal.h

@@ -45,7 +45,7 @@ void gl_R_Init (void);
 void glsl_R_Init (void);
 void sw_R_Init (void);
 void sw32_R_Init (void);
-void R_RenderFrame (SCR_Func scr_3dfunc, SCR_Func *scr_funcs);
+void R_RenderFrame (SCR_Func *scr_funcs);
 void R_Init_Cvars (void);
 void R_InitEfrags (void);
 void R_ClearState (void);