tr2/objects/xian: fix Xian warriors twitching in 60 fps

Resolves #2644.

src/libtrx/include/libtrx/game/items/types.h

@@ -69,6 +69,8 @@ typedef struct {
     uint16_t pad : 7; // 0x0200…0x8000
 #endif
 
+    bool enable_interpolation;
+
     struct {
         struct {
             XYZ_32 pos;

src/tr1/game/items.c

@@ -71,6 +71,7 @@ void Item_Initialise(int16_t item_num)
     item->priv = nullptr;
     item->carried_item = nullptr;
     item->enable_shadow = true;
+    item->enable_interpolation = true;
 
     if (item->flags & IF_INVISIBLE) {
         item->status = IS_INVISIBLE;

src/tr2/game/items.c

@@ -93,6 +93,7 @@ void Item_Initialise(const int16_t item_num)
     item->collidable = 1;
     item->looked_at = 0;
     item->killed = 0;
+    item->enable_interpolation = true;
 
     if ((item->flags & IF_INVISIBLE) != 0) {
         item->status = IS_INVISIBLE;
@@ -360,6 +361,7 @@ int32_t Item_GetFrames(const ITEM *item, ANIM_FRAME *frmptr[], int32_t *rate)
     // interpolated
     if (item != g_LaraItem
         && (!item->active || item->status != IS_ACTIVE
+            || !item->enable_interpolation
             || !Object_Get(item->object_id)->enable_interpolation)) {
         *rate = denominator;
         return numerator;

src/tr2/game/objects/creatures/xian_knight.c

@@ -12,6 +12,7 @@
 #include "global/vars.h"
 
 #include <libtrx/debug.h>
+#include <libtrx/game/interpolation.h>
 #include <libtrx/utils.h>
 
 // clang-format off
@@ -120,6 +121,7 @@ static void M_Control(const int16_t item_num)
     if (item->hit_points <= 0) {
         item->current_anim_state = XIAN_KNIGHT_STATE_DEATH;
         item->mesh_bits >>= 1;
+        item->enable_interpolation = false;
         if (item->mesh_bits == 0) {
             Sound_Effect(SFX_EXPLOSION_1, nullptr, SPM_NORMAL);
             item->mesh_bits = -1;

src/tr2/game/objects/creatures/xian_spearman.c

@@ -11,6 +11,7 @@
 #include "global/vars.h"
 
 #include <libtrx/debug.h>
+#include <libtrx/game/interpolation.h>
 #include <libtrx/utils.h>
 
 // clang-format off
@@ -157,6 +158,7 @@ static void M_Control(const int16_t item_num)
     if (item->hit_points <= 0) {
         item->current_anim_state = XIAN_SPEARMAN_STATE_DEATH;
         item->mesh_bits >>= 1;
+        item->enable_interpolation = false;
         if (item->mesh_bits == 0) {
             Sound_Effect(SFX_EXPLOSION_1, nullptr, SPM_NORMAL);
             item->mesh_bits = -1;