seekable_format: Prevent rereading frame when seeking forward

[YoniGilad]

When decompressing a seekable file, if seeking forward within
a frame (by issuing multiple ZSTD_seekable_decompress calls
with a small gap between them), the frame will be unnecessarily
reread from the beginning. This patch makes it continue using
the current frame data and simply skip over the unneeded bytes.

[Cyan4973]

While I understand the intention (reduce the nb of fseek()), it's unclear to me if this change is safe.

What happens when offset > zs->decompressedOffset ?
Why is it guaranteed to be always fine ?

Could there be a test which shows the benefit and correctness of the change ?

[YoniGilad]

@Cyan4973 When offset > zs->decompressedOffset (but still within the same frame), we will enter the if (zs->decompressedOffset < offset) condition below, which will cause it to decompress and skip the unneeded data. This is the same mechanism that skips from the start of a frame to the requested offset.

I will work on adding a test into seekable_tests.c that performs multiple reads at different offsets. To measure the benefit I think I can use the ZSTD_seekable_customFile mechanism with functions that keep track of the number of bytes read and then check that it's not too high.

[YoniGilad]

I added a test that does the following:

1. Compress test data into multiple frames
2. Perform a series of small decompressions and seeks forward, checking that the total data read is less than the compressed size - i.e. that compressed data wasn't reread. (this check fails without this PR's fix).
3. Perform some seeks forward and backward to ensure correctness.

[Cyan4973]

Thanks @YoniGilad ,
your patch has been rebased and re-proposed for merge in #3581 .