fixes #17 fix opening credits start time

[jmlich]

it seems opening credits was removed due to following condition:

        # Skip clips entirely before the start point
        if start + duration < self.start_time:
            return

Moving start time of opening credits helps

[valerio-bozzolan]

I tested this patch on my local computer and it just works as expected. Tried with:

./make-xges.py --opening-credits=intro-pepper.png asd presentation.xges

And it put my pepper in the intro as usual:

I recommend in merging this.

(I'm not the maintainer)

[valerio-bozzolan]

Premising that I've done some code review, and I'm merging what I like in this fork:

https://github.com/valerio-bozzolan/bbb-render

Hoping to be useful, this pull request was merged in that fork, at this point:

https://github.com/valerio-bozzolan/bbb-render/commit/53f9723d0cbd07a33a06610c4250dd58dde7accf