ShowFrameNumber scroll=false patch

##### ShowFrameNumber(scroll=false) patch

This patch addresses [bug #4](https://github.com/avxsynth/avxsynth/issues/4). Specifically, this patch corrects this bug:

- [x] "ShowFrameNumber does not create a wall of frame numbers but rather a single value."
- [ ] "font_width and font_angle parameters are ignored."
- [ ] "Text is placed top-left justified instead of center-justified when..."

I've tested this myself, and it seems to work. I used the code from `RenderShowFrameNumberScrolling` (with a few modifications) to keep the style consistent. NOTICE: My editor, vim, took the liberty to trim the excess whitespace.

This is my first contribution to AvxSynth. If I got something wrong, let me know. I am glad to help out however I can. Keep up the awesome work. It's about time we've got AviSynth for linux :]

Thanks