1.2.0

Before, KSE would crash if any sprites didn't exist.

Added new flag -S/--strict that restores the previous behaviour. Otherwise, KanimReader will insert a 1x1 transparent png for missing sprites, while the ScmlWriter will simply output an anim file that references a non-existent sprite. This feature is required because many Klei animations, especially critter and dupes ones, reference sprites like snapto_pivot_0 that are presumably used by the game to ensure animations appear in the right place relative to each other.