PartsEngine: Implement PartsFunc 3 (motion Pause/Resume)

Wire up PartsFunc 3 (PauseMotion) to a new PE_PauseMotion(). While
paused, PE_UpdateMotionTime stops advancing the motion clock.

include/parts.h

@@ -248,6 +248,7 @@ void PE_SeekEndMotion(void);
 void PE_UpdateMotionTime(int time, bool skip);
 bool PE_IsMotion(void);
 int PE_GetMotionEndTime(void);
+void PE_PauseMotion(bool pause);
 
 // text.c
 bool PE_SetText(int parts_no, struct string *text, int state);

src/hll/PartsEngine.c

@@ -260,6 +260,10 @@ static int PartsEngine_PartsFunc(int func_id, struct page **array_int,
 		REQUIRE_INTS(1);
 		ints[0].i = PE_get_system_controller();
 		return 1;
+	case 3:  // void PauseMotion(bool pause)
+		REQUIRE_INTS(1);
+		PE_PauseMotion(!!ints[0].i);
+		return 1;
 	case 4:  // void SetWantSave(int parts_no, bool want_save)
 		REQUIRE_INTS(2);
 		PE_parts_set_want_save(ints[0].i, !!ints[1].i);

src/parts/motion.c

@@ -38,6 +38,8 @@ static struct parts_motion_list global_motion_list =
 
 // true after call to BeginMotion until the motion ends or EndMotion is called
 static bool is_motion = false;
+// true while the motion is paused (time does not advance)
+static bool is_motion_paused = false;
 // the elapsed time of the current motion
 static int motion_t = 0;
 // the end time of the current motion
@@ -498,6 +500,7 @@ void PE_BeginMotion(void)
 	// FIXME: starting a motion seems to clear non-default states
 	motion_t = 0;
 	is_motion = true;
+	is_motion_paused = false;
 	parts_init_all_motion();
 }
 
@@ -508,6 +511,7 @@ void PE_EndMotion(void)
 	motion_t = 0;
 	motion_end_t = 0;
 	is_motion = false;
+	is_motion_paused = false;
 	parts_fini_all_motion();
 }
 
@@ -531,6 +535,8 @@ void PE_SeekEndMotion(void)
 void PE_UpdateMotionTime(int time, possibly_unused bool skip)
 {
 	// TODO: use skip
+	if (is_motion_paused)
+		return;
 	PE_SetMotionTime(motion_t + time);
 }
 
@@ -543,3 +549,20 @@ int PE_GetMotionEndTime(void)
 {
 	return motion_end_t;
 }
+
+void PE_PauseMotion(bool pause)
+{
+	if (parts_began_click)
+		return;
+	if (!is_motion)
+		return;
+	if (is_motion_paused == pause)
+		return;
+	is_motion_paused = pause;
+
+	// While paused, the per-frame advance stops updating the whole-screen
+	// vibration offset. Reset it to the origin so the screen doesn't stay
+	// stuck at the last shake position.
+	if (pause)
+		parts_set_global_pos((Point){ 0, 0 });
+}