Added check to backface culling to see if the face is behind the camera.

This will be required if we ditch depth-based culling in favor of leaf-based depth.
Also makes for a nice speed boost in some scenarios already.

common.h

@@ -22,6 +22,7 @@ typedef struct _STVECTOR {
 
 extern VECTOR cam_pos;
 extern SVECTOR cam_rot;
+extern SVECTOR cam_dir;
 extern u_short cam_leaf;
 
 extern u_short quake_clut;

input.c

@@ -34,6 +34,10 @@ void input_process()
 	MATRIX cam_mtx;
 	RotMatrixQ(&cam_rot, &cam_mtx);
 
+	cam_dir.vx = cam_mtx.m[0][1];
+	cam_dir.vy = cam_mtx.m[1][1];
+	cam_dir.vz = cam_mtx.m[2][1];
+
     int deltaTime = time_getDeltaTime();
     int moveInterval = (moveSpeed * deltaTime) >> 12;
     int rotInterval = (moveSpeed * deltaTime) >> 12;

main.c

@@ -11,6 +11,7 @@ extern u_long bsp_test[];
 VECTOR cam_pos = { 2176, 1152, 272 };	// START
 //VECTOR cam_pos = { 1920, -1408, 352 };	// E1M1
 SVECTOR cam_rot = { 0, 0, 0 };
+SVECTOR cam_dir = { 0, ONE, 0 };
 u_short cam_leaf = 0;
 
 world_t world;

world.c

@@ -78,6 +78,12 @@ static INLINE char world_cull_backface(const world_t *world, const ps1bsp_face_t
     cam_vec.vy = face->center.vy - cam_pos.vy;
     cam_vec.vz = face->center.vz - cam_pos.vz;
 
+    // Check if the face is behind the camera
+    short camDot = m_dot12s(cam_vec, cam_dir);
+    if (camDot < 0)
+        return 1;
+    
+    // Check if the face's plane points towards the camera
     const ps1bsp_plane_t *plane = &world->planes[face->planeId];
     *dot = world_planeDot(&cam_vec, plane);
     return ((*dot >= 0) ^ face->side);