mov: Prioritize aspect ratio values found in pasp atom

mov: Prioritize aspect ratio values found in pasp atom

From the ISO/IEC specification for MP4:
  The pixel aspect ratio and clean aperture of the video may be specified
  using the ‘pasp’ and ‘clap’ sample entry boxes, respectively. These are
  both optional; if present, they over-ride the declarations (if any) in
  structures specific to the video codec, which structures should be
  examined if these boxes are absent. For maximum compatibility, these
  boxes should follow, not precede, any boxes defined in or required by
  derived specifications.

Fixes trac/#7277.