Wrap tick API

This avoids to use the SDL timer API directly, and will allow to handle
generic ticks (possibly negative).

app/src/util/tick.h

@@ -0,0 +1,20 @@
+#ifndef SC_TICK_H
+#define SC_TICK_H
+
+#include <stdint.h>
+
+typedef int64_t sc_tick;
+#define SC_TICK_FREQ 1000000 // microsecond
+
+// To be adapted if SC_TICK_FREQ changes
+#define SC_TICK_TO_US(tick) (tick)
+#define SC_TICK_TO_MS(tick) ((tick) / 1000)
+#define SC_TICK_TO_SEC(tick) ((tick) / 1000000)
+#define SC_TICK_FROM_US(us) (us)
+#define SC_TICK_FROM_MS(ms) ((ms) * 1000)
+#define SC_TICK_FROM_SEC(sec) ((sec) * 1000000)
+
+sc_tick
+sc_tick_now(void);
+
+#endif