Update pm_shared.cpp

From ValveSoftware/halflife#3668

src/game/shared/movement/pm_shared.cpp

@@ -966,6 +966,16 @@ int PM_ClipVelocity(Vector &in, Vector &normal, Vector &out, float overbounce)
 			out[i] = 0;
 	}
 
+	// iterate once to make sure we aren't still moving through the plane
+	float adjust = DotProduct(out, normal);
+	if (adjust <= 0.0f)
+	{
+		// min this against a small number (but no further from zero than -DIST_EPSILON) to account for crossing a plane with a near-parallel normal
+		adjust = min(adjust, -DIST_EPSILON);
+		for (i=0 ; i<3 ; i++)
+			out[i] -= normal[i]*adjust;
+	}
+
 	// Return blocking flags.
 	return blocked;
 }