Add direct access to frame pict type in Frame Class

[steeveen]

https://github.com/bytedeco/javacv/issues/1729

[saudet]

Instead of using int or even enum, I was thinking we could just use char with as values I, P, B, and maybe others in the future. What do you think?

[steeveen]

Not a big problem.
char seems to be better for readability
But I saw the type contains SI、SP、BI

Using String to represent them might be not a appropriate way.

[saudet]

Looks like FFmpeg itself is just using lowercase letters in that case:
https://github.com/FFmpeg/FFmpeg/blob/master/libavutil/utils.c#L83-L95
So, let's just call av_get_picture_type_char() to get a char and put that in :)

[steeveen]

OK，you are right. I have converted it ~