OpenSystemsDevelopment / qpdf

When removing unreferenced resources, notice if a page (recursively)
contains a form XObject with unreferenced resources, and count any
such resources as referenced by the page.

Use the first of ASCII, PDFDocEncoding, or UTF-16 that is capable of
encoding the string.

Make some more methods in QPDFPageObjectHelper work with form
XObjects, provide forEach methods to walk through nested form
XObjects, possibly recursively. This should make it easier to work
with form XObjects from user code.

Turns out unreadCh is much more efficient than seek(-1, SEEK_CUR).
Update comments and code to reflect this.

Make clear that you almost always want + or - before an angle when
specifying rotation.

Issue #399 mentioned a use case for which qpdf has support, but the
fact that it is supported was not documented or in the test suite,
making it vulerable to accidental breakage.