engine: add new engine feature ENGINE_STEP_POSHISTORY_LERP that enables CStrike's MOVETYPE_STEP lerp based on position history

common/enginefeatures.h

@@ -27,6 +27,8 @@ GNU General Public License for more details.
 #define ENGINE_COMPUTE_STUDIO_LERP	(1<<7)	// enable MOVETYPE_STEP lerping back in engine
 #define ENGINE_LINEAR_GAMMA_SPACE	(1<<8)	// disable influence of gamma/brightness cvars to textures/lightmaps, for mods with custom renderer
 
+#define ENGINE_STEP_POSHISTORY_LERP (1U<<31) // enable MOVETYPE_STEP interpolation based on position history. Incompatible with ENGINE_COMPUTE_STUDIO_LERP!
+
 // adjust the mask when features will be added or removed
 #define ENGINE_FEATURES_MASK      \
 	( ENGINE_WRITE_LARGE_COORD    \
@@ -37,6 +39,7 @@ GNU General Public License for more details.
 	| ENGINE_COMPENSATE_QUAKE_BUG \
 	| ENGINE_IMPROVED_LINETRACE   \
 	| ENGINE_COMPUTE_STUDIO_LERP  \
-	| ENGINE_LINEAR_GAMMA_SPACE   )
+	| ENGINE_LINEAR_GAMMA_SPACE   \
+	| ENGINE_STEP_POSHISTORY_LERP )
 
 #endif//FEATURES_H

engine/client/cl_frame.c

@@ -189,7 +189,7 @@ CL_ParametricMove
 check for parametrical moved entities
 ==================
 */
-qboolean CL_ParametricMove( cl_entity_t *ent )
+static qboolean CL_ParametricMove( cl_entity_t *ent )
 {
 	float	frac, dt, t;
 	vec3_t	delta;
@@ -1279,7 +1279,10 @@ void CL_LinkPacketEntities( frame_t *frame )
 				if ( !CL_InterpolateModel( ent ))
 					continue;
 			}
-			else if( ent->curstate.movetype == MOVETYPE_STEP && !NET_IsLocalAddress( cls.netchan.remote_address ))
+			// a1ba: in GoldSrc this is done for cstrike and czero
+			// but let modders use this as an engine feature
+			else if( FBitSet( host.features, ENGINE_STEP_POSHISTORY_LERP ) &&
+				ent->curstate.movetype == MOVETYPE_STEP && !NET_IsLocalAddress( cls.netchan.remote_address ))
 			{
 				if( !CL_InterpolateModel( ent ))
 					continue;