* implement scrolling, follow playhead
 * ffmpeg vs. gtk image scaling.
 * fix grid colors, add labels, legend.
 * render frames directly into a pixmap 
   - no intermediate image-buffer.
   - flip pixmaps not buffers
   - threaded read-ahead.
 * video file reading (-ffmpeg-,gst)
 * config: cmd-line args, rc??
 * complete video-file attribute dialog (offsets)
 * compensate for latency and offset to jack.
 * allow to set scrub-buffer length (mark in timeline)
 * renice zoom behaviour, fix 1-frame zoom (offset)
 * set spinbtn page-increment accoding to samplerate & FPS!
 * lock update threads during user interaction/callbacks.
 * shut-down threads before exiting (prevent possible segfault @end)
 * gcc -O3 [-g] via configure/Makefile.am  opts