[alan-divd]

Summary

Reading to EOF on an OverlayStream with no known size used sys.maxsize as
the read length. That works on a BytesIO, but a real file object raises
OverflowError: byte string is too large.

Determine the actual end instead, by seeking the source to EOF and taking the
maximum of that and the end of every registered overlay, so an overlay that
extends past the end of the source stream is still fully read.

Fixes #114

Test plan

• tox run -e py3 passes (161 passed)
• The new test wraps a BytesIO in an io.BufferedReader so the
  OverflowError reproduces in memory, matching how the existing tests
  avoid touching the filesystem
• Covers an overlay that extends past the end of the source stream