SERVER: Disable FTE_PEXT_TRANS if client is outdated.

As outdated clients have a broken implementation of FTE_PEXT_TRANS
these will not read the PF_EXTRA_PFS byte and the network protocol
will go out of sync unless this extension is disabled.

Once disabled, the client will work just fine, just that transparent
entities will be opaque.

Another workaround is for the client to set cl_pext_alpha to 0.

src/sv_main.c

@@ -194,6 +194,10 @@ cvar_t sv_pext_mvdsv_serversideweapon = { "sv_pext_mvdsv_serversideweapon", "1"
 
 cvar_t sv_extlimits = { "sv_extlimits", "2" };
 
+#if defined(FTE_PEXT_TRANS)
+cvar_t sv_pext_ezquake_verfortrans = {"pext_ezquake_verfortrans", "7814", CVAR_NONE};
+#endif
+
 qbool sv_error = false;
 
 client_t *WatcherId = NULL; // QW262
@@ -3507,6 +3511,10 @@ void SV_InitLocal (void)
 	Cvar_Register (&sv_pext_mvdsv_serversideweapon);
 #endif
 
+#ifdef FTE_PEXT_TRANS
+	Cvar_Register(&sv_pext_ezquake_verfortrans);
+#endif
+
 	Cvar_Register (&sv_reliable_sound);
 
 	Cvar_Register(&qws_name);

src/sv_user.c

@@ -352,6 +352,33 @@ static void Cmd_New_f (void)
 	}
 #endif
 
+#if defined(FTE_PEXT_TRANS)
+	if (sv_client->fteprotocolextensions & FTE_PEXT_TRANS)
+	{
+		const char *client_string = Info_Get(&sv_client->_userinfo_ctx_, "*client");
+		char *ptr = strchr(client_string, ' ');
+		if (ptr != NULL) {
+			ptr++;
+			if (strncmp(client_string, "ezQuake", 7) == 0 && *ptr != '\0')
+			{
+				extern cvar_t sv_pext_ezquake_verfortrans;
+				char *endptr;
+				long revision = strtol(ptr, &endptr, 10);
+				if (*endptr != '\0' || (revision > 0 && revision < sv_pext_ezquake_verfortrans.value))
+				{
+					SV_ClientPrintf(sv_client, PRINT_HIGH, "\n\nWARNING:\n"
+						"Alpha support disabled due to buggy client, "
+						"if the map contains transparency you may be at a disadvantage.\n"
+						"Please upgrade to one of the following:\n"
+						"> ezQuake (https://www.ezquake.com)\n"
+						"> FTEQW (http://fte.triptohell.info/)\n");
+					sv_client->fteprotocolextensions &= ~FTE_PEXT_TRANS;
+				}
+			}
+		}
+	}
+#endif
+
 	//NOTE:  This doesn't go through ClientReliableWrite since it's before the user
 	//spawns.  These functions are written to not overflow
 	if (sv_client->num_backbuf)