consider adding keyframe support to tables to allow apng use for ship/weapon select

90b35b72fb245b64b9d977751893eeb8d0d77d69 says that apng's don't currently support keyframes thus they can't be used for ship & weapon select animations. However, it may be possible to add table entries for this instead of either hacking the apng format, creating a container format, or otherwise using some other png chunk type in the images... which probably causes issues with editors being able to _create_ said chunk correctly.

Anyway - this idea would add:.
+$Anim Keyframe: (weapons.tbl)
$Ship_Anim Keyframe: (ships.tbl)

Only issue (I can think of right now) is ensuring that when the keyframe is wanted, all the pixel data will be available. i.e. either the keyframe is a full image (not a delta), OR we're only skipping back to the keyframe, never forwards (thus we ensure that the relevant frame has already been composed from whatever the source is in the apng)