inventory: fix demo timer

Resolves LostArtefacts#1368. This is an improvement to b276946 which didn't address
the underlying issue. It also offers a small refactor to boost the
readability.

src/game/clock.c

@@ -40,7 +40,11 @@ static bool Clock_CheckElapsedUnit(
     const double multiplier =
         (bypass_turbo_cheat ? 1.0 : Clock_GetSpeedMultiplier()) / 1000.0;
     const double frames = delta * multiplier * unit;
-    if (frames >= how_often || g_Config.rendering.fps != timer->prev_fps) {
+    if (g_Config.rendering.fps != timer->prev_fps) {
+        Clock_ResetTimer(timer);
+        return false;
+    }
+    if (frames >= how_often) {
         Clock_ResetTimer(timer);
         return true;
     }

src/game/phase/phase_inventory.c

@@ -68,6 +68,7 @@ static int32_t InvItem_GetFrames(
     const INVENTORY_ITEM *inv_item, FRAME_INFO **out_frame1,
     FRAME_INFO **out_frame2, int32_t *out_rate);
 static void Inv_DrawItem(INVENTORY_ITEM *inv_item, int32_t frames);
+static bool Inv_CheckDemoTimer(const IMOTION_INFO *motion);
 
 static void Phase_Inventory_Start(void *arg);
 static void Phase_Inventory_End(void);
@@ -550,6 +551,23 @@ static void Inv_DrawItem(INVENTORY_ITEM *const inv_item, const int32_t frames)
     }
 }
 
+static bool Inv_CheckDemoTimer(const IMOTION_INFO *const motion)
+{
+    if (!g_Config.enable_demo || !g_GameFlow.has_demo) {
+        return false;
+    }
+
+    if (g_InvMode != INV_TITLE_MODE || g_Input.any || g_InputDB.any
+        || Console_IsOpened()) {
+        Clock_ResetTimer(&m_DemoTimer);
+        return false;
+    }
+
+    return motion->status == RNG_OPEN
+        && Clock_CheckElapsedMilliseconds(
+               &m_DemoTimer, g_GameFlow.demo_delay * 1000.0);
+}
+
 static void Phase_Inventory_Start(void *arg)
 {
     Interpolation_Remember();
@@ -657,19 +675,10 @@ static GAMEFLOW_COMMAND Phase_Inventory_ControlFrame(void)
     Inv_Ring_CalcAdders(ring, ROTATE_DURATION);
 
     Input_Update();
-
     // Do the demo inactivity check prior to postprocessing of the inputs.
-    if (g_InvMode != INV_TITLE_MODE || g_Input.any || g_InputDB.any
-        || Console_IsOpened()) {
-        Clock_ResetTimer(&m_DemoTimer);
-    } else if (g_Config.enable_demo && motion->status == RNG_OPEN) {
-        if (g_GameFlow.has_demo
-            && Clock_CheckElapsedMilliseconds(
-                &m_DemoTimer, g_GameFlow.demo_delay * 1000.0)) {
-            m_StartDemo = true;
-        }
+    if (Inv_CheckDemoTimer(motion)) {
+        m_StartDemo = true;
     }
-
     Shell_ProcessInput();
     Game_ProcessInput();