OpenSystemsDevelopment / qpdf

This code was essentially duplicated between test_driver and
standalone_fuzz_target_runner.

Have classes contain only a single private member of type
PointerHolder<Members>. This makes it safe to change the structure of
the Members class without breaking binary compatibility. Many of the
classes already follow this pattern quite successfully. This brings in
the rest of the class that are part of the public API.

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.

JSON serialization was preserving the value as presented, but JSON
doesn't accept decimal values without a 0 before the decimal point.

Support conversion of pages to form XObjects and placement of form
XObjects on pages.

Unparse is admittedly strange, but I'd rather be strange and
consistent, and everything else in the qpdf library uses unparse to
serialize. (If you're reading this, the convention of using "unparse"
comes from the "clu" programming language.)

Also move tests to libtests.

This fix allows qpdf to compile/test cleanly with gcc 8.

During periods of intensive operation on a specific file, this method
can reduce the overhead of repeated open/close operations.

ClosedFileInputSource is an input source that keeps the file closed
when not reading it.

Forgot to delete a buffer in the test driver. There was no memory leak
or memory error in any installed code.

Specify qpdf libraries before external ones.
Specify LDFLAGS before libraries.
This should eliminate remaining cases of qpdf builds favoring
previously installed versions.

Make sure to link from the source tree before linking from the system.
In many environments, this is necessary to allow a newly built qpdf to
link properly instead of trying to link or resolve libraries from an
older installed version.