Force gif aspect ratio multiplication to 64bit.

Avoids a possible integer overflow.
Show stream number when warning about stream without start time.
Allow .y as extension for image2.
Merge remote-tracking branch 'cehoyos/master'

* cehoyos/master:
  Allow .y as extension for image2.
  Show stream number when warning about stream without start time.
  Force gif aspect ratio multiplication to 64bit.

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