OpenSystemsDevelopment / qpdf

Put a specific comment marker next to every piece of code that MSVC
gives warning 4996 for.  This warning is generated for calls to
functions that Microsoft considers insecure or deprecated.  This
change is in preparation for fixing all these cases even though none
of them are actually incorrect or insecure as used in qpdf.  The
comment marker makes them easier to find so they can be fixed in
subsequent commits.

Make them safer by avoiding any internal limits and replacing sprintf
with std::ostringstream.

Add QUtil::hex_encode to encode binary data has a hexadecimal string,
and use it in place of sprintf where possible.

When caching objects in an object stream, only cache objects that
still resolve to that stream.  See Changelog mod from this commit for
details.

Fix exit status for case of errors without warnings, continue after
errors when possible, add test case for parsing a file with content
stream errors on some but not all pages.

Change object type Keyword to Operator, and place the order of the
object types in object_type_e in the same order as they are mentioned
in the PDF specification.

Note that this change only breaks backward compatibility with code
that has not yet been released.

Add virtual methods to QPDFObject, wrappers to QPDFObjectHandle, and
implementations to all the QPDF_Object types.

This method allows parsing of the PDF objects in a content stream or
array of content streams.

These object types are to facilitate content stream parsing.

Next released version will be 4.1.0 since new APIs are being added.

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.

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.

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.

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.