Use integer math instead of floats for timestamps

Author: asarium

code/freespace2/freespace.cpp

@@ -4871,7 +4871,7 @@ void game_set_frametime(int state)
 	Last_frame_timestamp = timestamp();
 
 	flFrametime = f2fl(Frametime);
-	timestamp_inc(flFrametime);
+	timestamp_inc(Frametime);
 
 	// wrap overall frametime if needed
 	if ( FrametimeOverall > (INT_MAX - F1_0) )

code/io/timer.cpp

@@ -214,9 +214,10 @@ void timestamp_reset()
 // something like 1 minute (6000).
 #define MAX_TIME (INT_MAX/2)
 
-void timestamp_inc(float frametime)
+void timestamp_inc(fix frametime)
 {
-	timestamp_ticker += (int)(frametime*TIMESTAMP_FREQUENCY);
+	auto frametime_ms = fixmul(frametime, F1_0 * TIMESTAMP_FREQUENCY);
+	timestamp_ticker += f2i(frametime_ms);
 
 	if ( timestamp_ticker > MAX_TIME )	{
 		timestamp_ticker = 2;		// Roll!

code/io/timer.h

@@ -73,7 +73,7 @@ extern int timestamp_ticker;
 extern void timestamp_reset();
 
 // Call this once every frame with the frametime.
-extern void timestamp_inc(float frametime);
+extern void timestamp_inc(fix frametime);
 
 // To do timing, call this with the interval you
 // want to check.  Then, pass this to timestamp_elapsed