Add FPS counter

Remove frame counter from scrcpy.c and add a new FPS counter, logging as
INFO the measured frame rate every second (on new frame).

app/src/fpscounter.h

@@ -0,0 +1,26 @@
+#ifndef FPSCOUNTER_H
+#define FPSCOUNTER_H
+
+#include <SDL2/SDL_stdinc.h>
+
+#include "config.h"
+
+struct fps_counter {
+    SDL_bool started;
+    Uint32 slice_start; // initialized by SDL_GetTicks()
+    int nr_rendered;
+#ifdef SKIP_FRAMES
+    int nr_skipped;
+#endif
+};
+
+void fps_counter_init(struct fps_counter *counter);
+void fps_counter_start(struct fps_counter *counter);
+void fps_counter_stop(struct fps_counter *counter);
+
+void fps_counter_add_rendered_frame(struct fps_counter *counter);
+#ifdef SKIP_FRAMES
+void fps_counter_add_skipped_frame(struct fps_counter *counter);
+#endif
+
+#endif