bitmap_packer: make manual use slightly more convenient

Apply the padding internally to each input bitmap, instead of requiring
this for the semi-public API.

Right now, everything still uses packer_pack_from_subbitmaps() to fill
the input bitmap sizes, but that's going to change with the following
commit. Since bitmap_packer.in is mutated during packing anyway, it's
more convenient to add the padding automatically.

Also, guarantee that every sub-bitmap has a padding border around it.
Don't let the padding overlap. Add padding even on the containing
borders.

This is simpler, doesn't cost much in memory usage, and is convenient
for one of the following commits.

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