Avoid accumulated rounding in call frame pacing

tools/chatgui/src/call_video_cadence.h

@@ -1,15 +1,17 @@
 #pragma once
-#include <algorithm>
 #include <cstdint>
 
 // Сетка 30 fps с допуском половины периода: дрожание PTS не превращает 30 fps в 15.
 class CallVideoCadence {
-    int64_t next = -1;
+    int64_t origin = -1, lastSlot = -1;
 public:
     bool accept(int64_t pts) {
-        constexpr int64_t period = 1000000 / 30;
-        if (next >= 0 && pts < next - period / 2) return false;
-        next = next < 0 ? pts + period : std::max(next + period, pts + period / 2);
+        if (origin < 0) origin = pts;
+        if (pts < origin) return false;
+        const int64_t elapsed = pts - origin;
+        const int64_t slot = (elapsed / 1000000) * 30 + ((elapsed % 1000000) * 30 + 500000) / 1000000;
+        if (slot <= lastSlot) return false;
+        lastSlot = slot;
         return true;
     }
 };