OpenSystemsDevelopment / qpdf

When checking two objects preceding R while parsing, ensure that the
objects are direct. This avoids stuff like 1 0 obj containing 1 0 R 0 R
from causing an infinite loop in object resolution.

Original reported here:
https://bugs.launchpad.net/ubuntu/+source/qpdf/+bug/1397413

The PDF specification says that the /Type key for nodes in the pages
dictionary (both /Page and /Pages) is required, but some PDF files
omit them. Use the presence of other keys to determine the type of
pages tree node this is if the type key is not found.

The spec allows /Contents to be omitted for pages that are blank, but
QPDFObjectHandle::getPageContents() was throwing an exception in this
case.

Without this, qpdf executables work only on Vista or newer.

Fixes #35

Used during creation of Windows releases

This is faster than using qpdf --pages to do it.

QPDFWriter was trying to make /Filter and /DecodeParms direct in all
cases, but there are some cases where /DecodeParms may refer to a
stream, which can't be direct. QPDFWriter doesn't actually need
/DecodeParms to be direct in that case because it won't be able to
filter the stream. Until we can handle this type of stream, just don't
make /Filter and /DecodeParms direct if we can't filter the stream
anyway.

Fixes #34

Fix problem: if the last object in the first part of a linearized file
had an offset that was below 65536 by less than the size of the hint
stream, the xref stream was invalid and the resulting file is not
usable.

This is a performance fix.  The output is unchanged.

Fixes #28.

Add a method to get the current random data provider, and document and
test the method for resetting it.

Also remove some trivial, non-functional code.

Add new RandomDataProvider object and implement existing random number
generation in terms of that.  This enables end users to supply their
own random data providers.

Fixes #27.

If NO_GET_ENVIRONMENT is #defined at compile time on Windows, do not
call GetEnvironmentVariable.  QUtil::get_env will always return
false.  This option is not available through configure.  This was
added to support a specific user's requirements to avoid calling
GetEnvironmentVariable from the Windows API.  Nothing in qpdf outside
the test coverage system in qtest relies on QUtil::get_env.

In compare image tests, use the gs device tiff24nc instead of tiff12nc
since the 4 bit per sample images created by tiff12nc could sometimes
trigger a bug in tiffcmp.  Fixes #20.

Also accept -accessibility=n with 256 bit keys even though it will be
ignored.

For std::string and std::vector, replace operator[] with at.  This was
done using an automated process.  See README.hardening for details.

If not available, give an error.  The user may also configure qpdf to
use an insecure random number generator.

Ideally, the library should never call assert outside of test code,
but it does in several places.  For some cases where the assertion
might conceivably fail because of a problem with the input data,
replace assertions with exceptions so that they can be trapped by the
calling application.  This commit surely misses some cases and
replaced some cases unnecessarily, but it should still be an
improvement.

In places where std::vector<T>(size_t) was used, either validate that
the size parameter is sane or refactor code to avoid the need to
pre-allocate the vector.

The /W array was not sanitized, possibly causing an integer overflow
in a multiplication. An analysis of the code suggests that there were
no possible exploits based on this since the problems were in checking
expected values but bounds checks were performed on actual values.

The faulty code was only used during explicit checks of linearization
data.  Those checks are not part of normal reading or writing of PDF
files.