Ignoring sample aspect ratio (SAR) when produce thumbnails

[tim-lansen]

The thumbnail capture initialization procedure ignores sample aspect ratio of the input frame.
It leads to wrong output display aspect when i request a thumb without dimensions or one dim set.

For example, the source video has
width=1920
height=1080
sample_aspect_ratio=9:16
the display aspect ratio (DAR) in this case is 1:1

but if i request thumb-1000.jpg or thumb-1000-h1080.jpg, i get picture with w=1920, h=1080, SAR=1:1, DAR=16:9
the correct dimensions in this case must be w=1080, h=1080 (DAR=1:1)