engine: introduce bug compatibility levels

* for now we only have GoldSrc bug compatibility, can be used for
  games that require precise GoldSrc behaviour, like CSCZDS
* enabled with -bugcomp command line
* added text in --help

engine/common/common.h

@@ -307,6 +307,16 @@ typedef struct
 	float		scale;		// curstate.scale
 } tentlist_t;
 
+typedef enum bugcomp_e
+{
+	// default mode, we assume that user dlls are not relying on engine bugs
+	BUGCOMP_OFF,
+
+	// GoldSrc mode, user dlls are relying on GoldSrc specific bugs
+	// but fixing them may break regular Xash games
+	BUGCOMP_GOLDSRC,
+} bugcomp_t;
+
 typedef struct host_parm_s
 {
 	HINSTANCE			hInst;
@@ -381,6 +391,9 @@ typedef struct host_parm_s
 	struct decallist_s	*decalList;	// used for keep decals, when renderer is restarted or changed
 	int		numdecals;
 
+	// bug compatibility level, for very "special" games
+	bugcomp_t bugcomp;
+
 } host_parm_t;
 
 extern host_parm_t	host;

engine/common/host.c

@@ -151,6 +151,8 @@ void Sys_PrintUsage( void )
 	O("-clientlib <path>","override client DLL path")
 #endif
 	O("-rodir <path>    ","set read-only base directory, experimental")
+	O("-bugcomp         ","enable precise bug compatibility. Will break games that don't require it")
+	O("                 ","Refer to engine documentation for more info")
 
 	O("-ip <ip>         ","set custom ip")
 	O("-port <port>     ","set custom host port")
@@ -941,6 +943,13 @@ void Host_InitCommon( int argc, char **argv, const char *progname, qboolean bCha
 	// member console allowing
 	host.allow_console_init = host.allow_console;
 
+	if( Sys_CheckParm( "-bugcomp" ))
+	{
+		// add argument check here when we add other levels
+		// of bugcompatibility
+		host.bugcomp = BUGCOMP_GOLDSRC;
+	}
+
 	// timeBeginPeriod( 1 ); // a1ba: Do we need this?
 
 	// NOTE: this message couldn't be passed into game console but it doesn't matter