OpenSystemsDevelopment / qpdf

Fixed spelling errors in previously published commits and update
spelling dictionary

When preparing the trailer for writing to the new file, trim a copy of
the trailer instead of the original file's trailer.

The upcoming 3.1 release contains non-compatible API changes, though
they only affect parts of the interface that are extremely unlikely to
have been used outside of qpdf itself.  The methods and data types
affected were used for communication between QPDFWriter and QPDF and
would have had no real use in end user code.

Also add copyright notice to a few public headers that were missing
one.

When writing a new stream, always remove /Crypt even if we are not
otherwise able to filter the stream.

Original code was written before we could shallow copy objects, so all
the filtering was done by suppressing the output of certain keys and
replacing them with other keys.  Now we can simplify the code greatly
by modifying shallow copies of dictionaries in place.

Read and write support is implemented for /V=5 with /R=5 as well as
/R=6.  /R=5 is the deprecated encryption method used by Acrobat IX.
/R=6 is the encryption method used by PDF 2.0 from ISO 32000-2.

Test cases added in a future commit since they depend on /R=6 support.

Change variable name for better clarity.

I noticed a test output file that was not accessed in the test suite
and added a test case for it.

Implemented pipeline around sph sha calls using standard test vectors
for full-byte values.  Did not test or support partial byte values.

Changes from upstream are limited to change #include paths so that I
can place header files and included "c" files in a subdirectory.  I
didn't keep the unit tests from sphlib but instead verified them by
running them manually.  I will implement the same tests using the
Pl_SHA2 pipeline except that sphlib's sha2 implementation supports
partial bytes, which I will not exercise in qpdf or our tests.

All version operations are now fully aware of extension levels.

This file used to exercise a zero offset test case when qpdf would
visit every object in the file.  After the next commit, qpdf no longer
touches unreferenced objects, so a reference had to be added to
continue to have this file exercise the zero offset case.

For linearization tests where we are actually comparing the exact
output of the test with a known file, uncompress stream data so we can
see what's there.  This makes looking at future changes a little
easier.

Allowing users to subclass InputSource and Pipeline to read and write
from/to arbitrary sources provides the maximum flexibility for users
who want to read and write from other than files or memory.

Since sourceforce automatically displays README-what-to-download.txt,
it is no longer necessary for this to be the default file on
Windows.  Instead make one of the binary distributions the default on
Windows.