Extract current video_buffer to frame_buffer

The current video buffer only stores one pending frame.

In order to add a new buffering feature, move this part to a separate
"frame buffer". Keep the video_buffer, which currently delegates all its
calls to the frame_buffer.

app/src/frame_buffer.c

@@ -0,0 +1,88 @@
+#include "frame_buffer.h"
+
+#include <assert.h>
+#include <libavutil/avutil.h>
+#include <libavformat/avformat.h>
+
+#include "util/log.h"
+
+bool
+sc_frame_buffer_init(struct sc_frame_buffer *fb) {
+    fb->pending_frame = av_frame_alloc();
+    if (!fb->pending_frame) {
+        return false;
+    }
+
+    fb->tmp_frame = av_frame_alloc();
+    if (!fb->tmp_frame) {
+        av_frame_free(&fb->pending_frame);
+        return false;
+    }
+
+    bool ok = sc_mutex_init(&fb->mutex);
+    if (!ok) {
+        av_frame_free(&fb->pending_frame);
+        av_frame_free(&fb->tmp_frame);
+        return false;
+    }
+
+    // there is initially no frame, so consider it has already been consumed
+    fb->pending_frame_consumed = true;
+
+    return true;
+}
+
+void
+sc_frame_buffer_destroy(struct sc_frame_buffer *fb) {
+    sc_mutex_destroy(&fb->mutex);
+    av_frame_free(&fb->pending_frame);
+    av_frame_free(&fb->tmp_frame);
+}
+
+static inline void
+swap_frames(AVFrame **lhs, AVFrame **rhs) {
+    AVFrame *tmp = *lhs;
+    *lhs = *rhs;
+    *rhs = tmp;
+}
+
+bool
+sc_frame_buffer_push(struct sc_frame_buffer *fb, const AVFrame *frame,
+                  bool *previous_frame_skipped) {
+    sc_mutex_lock(&fb->mutex);
+
+    // Use a temporary frame to preserve pending_frame in case of error.
+    // tmp_frame is an empty frame, no need to call av_frame_unref() beforehand.
+    int r = av_frame_ref(fb->tmp_frame, frame);
+    if (r) {
+        LOGE("Could not ref frame: %d", r);
+        return false;
+    }
+
+    // Now that av_frame_ref() succeeded, we can replace the previous
+    // pending_frame
+    swap_frames(&fb->pending_frame, &fb->tmp_frame);
+    av_frame_unref(fb->tmp_frame);
+
+    if (previous_frame_skipped) {
+        *previous_frame_skipped = !fb->pending_frame_consumed;
+    }
+    fb->pending_frame_consumed = false;
+
+    sc_mutex_unlock(&fb->mutex);
+
+    return true;
+}
+
+void
+sc_frame_buffer_consume(struct sc_frame_buffer *fb, AVFrame *dst) {
+    sc_mutex_lock(&fb->mutex);
+    assert(!fb->pending_frame_consumed);
+    fb->pending_frame_consumed = true;
+
+    av_frame_move_ref(dst, fb->pending_frame);
+    // av_frame_move_ref() resets its source frame, so no need to call
+    // av_frame_unref()
+
+    sc_mutex_unlock(&fb->mutex);
+}