img2enc: add an option for overwriting one file with subsequent images

Based on a patch by Michael Niedermayer <michaelni@gmx.at>.
Cosmetics, restore alphabetic order in atomics lists.
doc: document the prores encoder options

Adapted from the author's blag.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Signed-off-by: Anton Khirnov <anton@khirnov.net>