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.c

@@ -0,0 +1,16 @@
+#include "tick.h"
+
+#include <SDL2/SDL_timer.h>
+
+sc_tick
+sc_tick_now(void) {
+    // SDL_GetTicks() resolution is in milliseconds, but sc_tick are expressed
+    // in microseconds to store PTS without precision loss.
+    //
+    // As an alternative, SDL_GetPerformanceCounter() and
+    // SDL_GetPerformanceFrequency() could be used, but:
+    //  - the conversions (avoiding overflow) are expansive, since the
+    //    frequency is not known at compile time;
+    //  - in practice, we don't need more precision for now.
+    return (sc_tick) SDL_GetTicks() * 1000;
+}