[Cin] removing letter boxing while preserving aspect

[Andrea paz]

Actually in the case of my png image, the "Resize track" step is not
necessary, but it is always better to have the project format the same
as the track format in Timeline, especially if you are using multiple
clips.

Note: "Set Format" and "Resize track" cause a crop, which is necessary
in your case, but we often want to avoid it by using the "Scale"
plugin instead of "Resize track."

@IgorBeg can you please indicate your method? (I plan to make an
addition to the manual).