WIA: Reuse groups when writing

This is useful for the way Dolphin scrubs Wii discs.
The encrypted data is what gets zeroed out, but this
zeroed out data then gets decrypted before being stored,
and the resulting data does not compress well.
However, each block of decrypted scrubbed data is
identical given the same encryption key, and there's
nothing stopping us from making multiple group entries
point to the same offset in the file, so we only have
to store one copy of this data per partition.

For reference, wit zeroes out the decrypted data,
but Dolphin's WIA writer can't do this because it currently
doesn't know which parts of the disc are scrubbed.

This is also useful for things such as storing Datel discs
full of 0x55 blocks (repesenting unreadable blocks)
without compression enabled.

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