Most of the time, the frame resolution is unchanged, and the scaler only
needs to convert pixel formats.

Also avoids this crash when size is the same: https://trac.ffmpeg.org/ticket/5356



Change-Id: I4c1c3e0a8b5bba8c937317074e8a9c5652fca407
Gitlab: #58
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>