OpenSystemsDevelopment / qpdf

There are no reasons other than historical to use size_t.
On balance, using map is more efficient. Hold shared pointers to
QPDFObjects rather than QPDFObjectHandles for consistencey with
QPDF_Array.

Also, add const overload of QPDFObjectHandle::getObj

Stop using nullptr to represent null objects.
Count null array elements and trigger creation of sparse arrays if null count is
greater than 100.

Add temporary clone of SparseOHArray  to implement non-sparse mode.

Add optional parameter shallow. Change logic errors to runtime errors.

QPDFValueProxy wasn't a good name for it. We decided the evil of
having the header file be named QPDFObject_private.hh was less than
the evil of having the class be named something other than what it
should have been named.

I decided that it's actually fine to copy a direct object to another
QPDF. Even if we eventually prevent a QPDFObject from having multiple
parents, this could happen if an object is moved.

This is in preparation for restoring a QPDFObject.hh to ease the
transition on qpdf_object_type_e.

This commit was created by
* Renaming QPDFObject.cc and QPDFObject.hh
* Replacing QPDFObject\b with QPDFValueProxy (where \b is word
  boundary)
* Running format-code
* Manually resorting files in libqpdf/CMakeLists.txt
* Manually refilling the comment in QPDF.hh near class Resolver

On destruction of the QPDF object replace all indirect object references
with direct nulls.

Remove all existing code to release resolved references.

Fixes performance issue due to interaction of resetting QPDFValue::qpdf and
og members and prior code.

Move responsibility for creating shared pointers to objects and cloning from QPDFObjectHandle to QPDFObject.

Update getJSON() methods and calls to them

Where not possible, use "auto" to get the iterator type.

Editorial note: I have avoid this change for a long time because of
not wanting to make gratuitous changes to version history, which can
obscure when certain changes were made, but with having recently
touched every single file to apply automatic code formatting and with
making several broad changes to the API, I decided it was time to take
the plunge and get rid of the older (pre-C++11) verbose iterator
syntax. The new code is just easier to read and understand, and in
many cases, it will be more effecient as fewer temporary copies are
being made.

m-holger, if you're reading, you can see that I've finally come
around. :-)

Change .clang-format and commit automated changes from a fresh run of
format-code

Run this:

for i in  **/*.cc **/*.c **/*.h **/*.hh; do
  clang-format < $i >| $i.new && mv $i.new $i
done

This comment expands all tabs using an 8-character tab-width. You
should ignore this commit when using git blame or use git blame -w.

In the early days, I used to use tabs where possible for indentation,
since emacs did this automatically. In recent years, I have switched
to only using spaces, which means qpdf source code has been a mixture
of spaces and tabs. I have avoided cleaning this up because of not
wanting gratuitous whitespaces change to cloud the output of git
blame, but I changed my mind after discussing with users who view qpdf
source code in editors/IDEs that have other tab widths by default and
in light of the fact that I am planning to start applying automatic
code formatting soon.

Also switch to colon-style iteration in some cases. Thanks to Dean
Scarff for drawing this to my attention after detecting some
unnecessary copies with
https://clang.llvm.org/extra/clang-tidy/checks/performance-for-range-copy.html

This makes all integer type conversions that have potential data loss
explicit with calls that do range checks and raise an exception. After
this commit, qpdf builds with no warnings when -Wsign-conversion
-Wconversion is used with gcc or clang or when -W3 -Wd4800 is used
with MSVC. This significantly reduces the likelihood of potential
crashes from bogus integer values.

There are some parts of the code that take int when they should take
size_t or an offset. Such places would make qpdf not support files
with more than 2^31 of something that usually wouldn't be so large. In
the event that such a file shows up and is valid, at least qpdf would
raise an error in the right spot so the issue could be legitimately
addressed rather than failing in some weird way because of a silent
overflow condition.

Give objects descriptions and context so it is possible to issue
warnings instead of fatal errors for attempts to access objects of the
wrong type.

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