Warn if rawvideo and an unreadable pix_fmt are written.

Print an error if a combination of rawvideo and an unusual pix_fmt
that will be impossible to decode are written to avi or mov.
Fixes ticket #3545.
Improve amr bitrate calculation for VBR files.

Fixes ticket #3541.
Merge remote-tracking branch 'cehoyos/master'

* cehoyos/master:
  Improve amr bitrate calculation for VBR files.
  Warn if rawvideo and an unreadable pix_fmt are written.

Merged-by: Michael Niedermayer <michaelni@gmx.at>