resample-dpi: use long form for flags

[vitorgalvao]

Short flags are great when typing a command, but are less useful in a script, where comprehension is more important than speed.

Perhaps a matter of preference, but I’d say when you can understand what a command is doing without having to open the man page and check every flag because they consist of a single letter, you have a better experience, and might be more inclined to fix bugs if an outsider to the code.

[vitorgalvao]

Closing for lack of interest, as part of personal cleanup of old pull requests and issues.