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/meson.build

@@ -10,6 +10,7 @@ src = [
     'src/event_converter.c',
     'src/file_handler.c',
     'src/fps_counter.c',
+    'src/frame_buffer.c',
     'src/input_manager.c',
     'src/opengl.c',
     'src/receiver.c',