Disable renderswaps when maps are being rendered

Should hopefully prevent that obnoxious renderer swap SIGSEGV from ever being run into

src/d_main.cpp

@@ -594,11 +594,15 @@ static void D_Display(void)
 	// STUPID race condition...
 	{
 		wipegamestate = gamestate;
+		cansetrender = true;
 
 		// clean up border stuff
 		// see if the border needs to be initially drawn
 		if (gamestate == GS_LEVEL || (gamestate == GS_TITLESCREEN && titlemapinaction && curbghide && (!hidetitlemap)))
 		{
+			// Rendering a level; don't allow renderer changes!
+			cansetrender = false;
+
 			D_RenderLevel();
 
 			ps_uitime = I_GetPreciseTime();

src/screen.c

@@ -51,6 +51,7 @@
 viddef_t vid;
 INT32 setmodeneeded; //video mode change needed if > 0 (the mode number to set + 1)
 UINT8 setrenderneeded = 0;
+boolean cansetrender = true;
 
 //added : 03-02-98: default screen mode, as loaded/saved in config
 consvar_t cv_scr_width = CVAR_INIT ("scr_width", "640", CV_SAVE, CV_Unsigned, NULL);
@@ -496,6 +497,13 @@ void SCR_ChangeRenderer(void)
 	|| (signed)rendermode == cv_renderer.value)
 		return;
 
+	if (!cansetrender)
+	{
+		CONS_Alert(CONS_NOTICE, "You cannot change renderers at the moment.\n");
+		CV_SetValue(&cv_renderer, (signed)rendermode);
+		return;
+	}
+
 #ifdef HWRENDER
 	// Check if OpenGL loaded successfully (or wasn't disabled) before switching to it.
 	if ((vid.glstate == VID_GL_LIBRARY_ERROR)

src/screen.h

@@ -88,6 +88,7 @@ enum
 extern viddef_t vid;
 extern INT32 setmodeneeded; // mode number to set if needed, or 0
 extern UINT8 setrenderneeded;
+extern boolean cansetrender;
 
 extern double averageFPS;