Make SKIP_FRAMES a compilation flag

The skip_frames flag was a non-configurable runtime flag. Since it is
not exposed to the user, there is no need for a (possible) runtime cost.

For testing purpose, we still want it to be configurable, so make it a
compilation flag.

app/src/frames.h

@@ -4,6 +4,8 @@
 #include <SDL2/SDL_mutex.h>
 #include <SDL2/SDL_stdinc.h>
 
+#include "config.h"
+
 // forward declarations
 typedef struct AVFrame AVFrame;
 
@@ -11,7 +13,9 @@ struct frames {
     AVFrame *decoding_frame;
     AVFrame *rendering_frame;
     SDL_mutex *mutex;
+#ifndef SKIP_FRAMES
     SDL_cond *rendering_frame_consumed_cond;
+#endif
     SDL_bool rendering_frame_consumed;
 };