Rename "frames" to "video_buffer"

It better describes the purpose of the structure.

app/src/video_buffer.h

@@ -0,0 +1,42 @@
+#ifndef VIDEO_BUFFER_H
+#define VIDEO_BUFFER_H
+
+#include <SDL2/SDL_mutex.h>
+#include <SDL2/SDL_stdinc.h>
+
+#include "config.h"
+#include "fps_counter.h"
+
+// forward declarations
+typedef struct AVFrame AVFrame;
+
+struct video_buffer {
+    AVFrame *decoding_frame;
+    AVFrame *rendering_frame;
+    SDL_mutex *mutex;
+#ifndef SKIP_FRAMES
+    SDL_bool stopped;
+    SDL_cond *rendering_frame_consumed_cond;
+#endif
+    SDL_bool rendering_frame_consumed;
+    struct fps_counter fps_counter;
+};
+
+SDL_bool video_buffer_init(struct video_buffer *vb);
+void video_buffer_destroy(struct video_buffer *vb);
+
+// set the decoded frame as ready for rendering
+// this function locks frames->mutex during its execution
+// returns true if the previous frame had been consumed
+SDL_bool video_buffer_offer_decoded_frame(struct video_buffer *vb);
+
+// mark the rendering frame as consumed and return it
+// MUST be called with frames->mutex locked!!!
+// the caller is expected to render the returned frame to some texture before
+// unlocking frames->mutex
+const AVFrame *video_buffer_consume_rendered_frame(struct video_buffer *vb);
+
+// wake up and avoid any blocking call
+void video_buffer_stop(struct video_buffer *vb);
+
+#endif