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.

Warn if catalog type entry is invalid (fixes #810)

Improve --optimize-images to find images nested within XObjects

Refactor QdfFixer::processLines

Remove redundant parameter depth from QPDF::updateObjectMaps

Add new data member QPDFWriter::Members::root_og

Also, uninitialize QPDFWriter::Members members.

Also, change QPDFJob to use unique_ptr<QPDF> instead of shared pointers.