Videos: Avoid float math for frame durations

SMK format actually defines frame durations in units that are 0.01ms:

https://github.com/diasurgical/libsmackerdec/blob/91e732bb6953489077430572f43fc802bf2c75b2/src/SmackerDecoder.cpp#L329-L334

That's great because it means we can do everything using integer math.

Source/storm/storm_svid.cpp

@@ -31,15 +31,33 @@ std::uint8_t SVidAudioDepth;
 std::unique_ptr<int16_t[]> SVidAudioBuffer;
 #endif
 
+// Smacker's atomic time unit is a one hundred thousand's of a second (i.e. 0.01 millisecond, or 10 microseconds).
+// We use SDL ticks for timing, which have millisecond resolution.
+// There are 100 Smacker time units in a millisecond.
+constexpr uint64_t SmackerTimeUnit = 100;
+constexpr uint64_t TimeMsToSmk(uint64_t ms) { return ms * SmackerTimeUnit; }
+constexpr uint64_t TimeSmkToMs(uint64_t time) { return time / SmackerTimeUnit; };
+uint64_t GetTicksSmk()
+{
+#if SDL_VERSION_ATLEAST(2, 0, 18)
+ return TimeMsToSmk(SDL_GetTicks64());
+#else
+ return TimeMsToSmk(SDL_GetTicks());
+#endif
+}
+
 uint32_t SVidWidth, SVidHeight;
-double SVidFrameEnd;
-double SVidFrameLength;
 bool SVidLoop;
 SmackerHandle SVidHandle;
 std::unique_ptr<uint8_t[]> SVidFrameBuffer;
 SDLPaletteUniquePtr SVidPalette;
 SDLSurfaceUniquePtr SVidSurface;
 
+// The end of the current frame (time in SMK time units from the start of the program).
+uint64_t SVidFrameEnd;
+// The length of a frame in SMK time units.
+uint32_t SVidFrameLength;
+
 bool IsLandscapeFit(unsigned long srcW, unsigned long srcH, unsigned long dstW, unsigned long dstH)
 {
  return srcW * dstH > dstW * srcH;
@@ -278,7 +296,10 @@ bool SVidPlayBegin(const char *filename, int flags)
  }
 #endif
 
- SVidFrameLength = 1000000.0 / Smacker_GetFrameRate(SVidHandle);
+ // SMK format internally defines the frame rate as the frame duration
+ // in either milliseconds or SMK time units (0.01ms). The library converts it
+ // to FPS, which is always an integer, and here we convert it back to SMK time units.
+ SVidFrameLength = 100000 / static_cast<uint32_t>(Smacker_GetFrameRate(SVidHandle));
  Smacker_GetFrameSize(SVidHandle, SVidWidth, SVidHeight);
 
 #ifndef USE_SDL1
@@ -318,7 +339,7 @@ bool SVidPlayBegin(const char *filename, int flags)
  SVidPalette = SDLWrap::AllocPalette();
  UpdatePalette();
 
- SVidFrameEnd = SDL_GetTicks() * 1000.0 + SVidFrameLength;
+ SVidFrameEnd = GetTicksSmk() + SVidFrameLength;
 
  return true;
 }
@@ -329,7 +350,7 @@ bool SVidPlayContinue()
  UpdatePalette();
  }
 
- if (SDL_GetTicks() * 1000.0 >= SVidFrameEnd) {
+ if (GetTicksSmk() >= SVidFrameEnd) {
  return SVidLoadNextFrame(); // Skip video and audio if the system is to slow
  }
 
@@ -345,16 +366,16 @@ bool SVidPlayContinue()
  }
 #endif
 
- if (SDL_GetTicks() * 1000.0 >= SVidFrameEnd) {
+ if (GetTicksSmk() >= SVidFrameEnd) {
  return SVidLoadNextFrame(); // Skip video if the system is to slow
  }
 
  if (!BlitFrame())
  return false;
 
- double now = SDL_GetTicks() * 1000.0;
+ uint64_t now = GetTicksSmk();
  if (now < SVidFrameEnd) {
- SDL_Delay(static_cast<Uint32>((SVidFrameEnd - now) / 1000.0)); // wait with next frame if the system is too fast
+ SDL_Delay(static_cast<Uint32>(TimeSmkToMs(SVidFrameEnd - now))); // wait with next frame if the system is too fast
  }
 
  return SVidLoadNextFrame();